Dating mens vintage suits can be quick & easy if you use the following guidelines:

1.   The first thing is check the pants closure to see if it is a button fly, metal zip or Nylon zip.

  A button fly will usually date to anytime before the mid 40's;  zippers started being used in mens trousers in 1937, but were costly to install.  You will find button fly closures on WWII military trousers as well as mens suits and slacks until about the end of the War.

To qualify the button fly info above:  many men have suits tailored (bespoke) with button flys and it was also a style trend in the 60's & 70's.  Button flys on trousers up thru the late 30's would have a button closure to the waistband;  large hook & bar closures for the waistband didn't appear until into WWII. 

An invisible zipper was patented in 1940 and used in mens trousers, but they are rare to find.  The usual metal zipper will overlap the War years and be used consistently until the mid 60's when the Nylon zipper came into vogue. There is a transition / overlap period between metal & Nylon zippers.  The late 70's & 80's trended toward a 40's retro style and brought back metal zips to some extent, and some mens suit companies have never used anything but metal zips in their suit trousers. 

2.   Check the belt loops on the trousers.  The actual loops should be about 1/4" wide for any Pre 60's trousers going to as wide as 1" in the 70's. 

  The top pair shows a Hollywood style pant which was constructed with no sewn on waistband.  Hollywood pants appeared in the 30's with no belt loops, but are usually shown from the 40s on with drop loops - meaning the top of the belt loops started / dropped about 1/4" to 1/2" from the top of the waist.  This style gained even more popularity after the War and stayed until the mid 50's.  Note that in the late 40s and early 50s pants also had a sewn on waistband with dropped loops, know as modified Hollywood style, or a sewn on waistband with regular placed loops.

If the pants have braces buttons, they should be on the inside of the waistband.  Outside placed braces buttons usually denote Victorian, contemporary 90s, or bespoke.

3.   Check the inside right interior jacket pocket for an ACWA (Amalgamated Clothing Workers of America) Union Label.

  There are 4 basic dated ACWA labels:  1915 issue was used until 1936;  1936 issue was used until 1939;  1939 issue was used until 1949; and the 1949 issue was used as late as 1979 (that I've found).   The issue dates are written on the labels, but are tiny and sometimes hard to read.  There are also 2 or 3 letter designations on the labels that can be used to more closely date the garment, but I won't get into that info on this dating guide. 

The issue date will be the oldest a garment can be, but the garment will usually be newer than the issue date.

An ACTWU label dates from 1975 (with the merger of the above union and the Textile Workers Union of America) until 1995.

A UNITE label dates from 1995  (with the merger of ACTWU and ILGWU).

4.  Check the way that the jacket is lined.   Although some jackets were completely lined in the 20's, 30's, and 40's, especially in formalwear, the usual lining was "petals" at the shoulders with the front pieces and arms completely lined.  Also note that the seam edges are bound in fabric, not just turned under and pressed. 

   Petal linings were used until the early 50's when linings started going completely across the shoulder and were about 1/3 of the length of the jacket. 

5.   Look at the size of the lapels.  Throughout the decades, lapels have been a common width of 3" to 4" wide.  But then you have the exceptions: 

   wide: (5"+) usually on a peak lapel of a double breasted jacket and was popular about 1919-1920, tagain gained popularity from the 30's into the mid 50's.  Wide lapels reappeared in the late 60's into the 70's, and after a short repreve came to popularity in the late 90's. 

  And the skinny: 2" lapels made their first short lived appearance in the very early 20's, and would not reappear until the mid 50's when they stayed popular for nearly a decade.   They also made a quick appearance in the 80's.  Will the look reemerge with the popularity of Mad Men?

And I'll make a quick point about collarless jackets:  High button, collarless jackets first appeared about 1960, but the Beatles first tour of the US in the early 60's brought them to the forefront of mens fashion.  They reappeared in the mid 70's with lower mid chest buttons.

Nehru jackets appeared in the US about 1969-70 and went away as fast as they came on the fashion scene. 

6.   Look at the buttonholes. 

  Keyhole buttonholes (looking like a skeleton key should fit into them) are classic for mens suits from the 20's into the 60's at which time the sewn buttonholes changed to more of a loop than a keyhole.  Tailored buttonholes (all fabric not just sewn), like found on coats, are more time consuming and tedious to make and weren't used very often until about the 70's.

7.  And, finally, a few more quick clues: 

VENTS:  Some jackets had deep vents to the waist for horse riding thru the mid 30's, but many had no vents.  By the late 30's, double side vents or no vents were the norm.  No vents continued until the early 50's with short 5-6" back vents appeared.  By the mid to late 60's back vents were 11-12" long - again going almost to the waist, and the mid 70's vents shorted back to about 7-8". 

FABRIC:  If the suit is Polyester or a Polyester blend, it can be no earlier than the mid 50's.

  The International ID for Wool /  Woolmark label (swirly thing) dates the suit to after 1964.

  Garment care labels will usually date to after 1971 when they became mandatory in the US.  Note that manufacturers used Dry Clean or Dry Clean Only labels as early as the 30's.   International garment care symbols date to 1997 for US clothing, although they were used many years before that in Europe and Canada.

OTHER LABELS:  The brand label / selling company on the outside of the inside chest pocket can also yield clues;   Penney's used more than 6 distinctive fonts / logos over the years which can be looked up in my Penneys dating guides for accurate dating. 

Other company labels might be available on the Vintage Fashion Guild website.  

A department store / mens store label can also yield clues to dating.  Do not confuse a store label with a company manufacturing label.  Hart, Schaffner, Marx is a manufacturing label, while Macys is a store or possibly store brand label.  If you know a store went out of business in 1963, then the suit has to pre-date that. This info can sometimes be found in old newspaper ads and yearly city directory listings.   

Any label in the back neck that is used as a hang loop or where the hang loop would be, whether it be Brooks Bros., Dry Clean Only, or Made in Canada, usually denotes 1970's or newer.  This does not include labels placed in the back center at shoulder level - like the placement of Burberrys labels.

FIT / MEASURING:  most vintage pre-60s suits are not marked as to size.  To ascertain this:  jacket is to be buttoned and laid out smoothly on a flat surface.  Then measure from armpit to armpit.  As a rule of thumb, subtract 3-4" for lining, clothing and movement to get a size.  So a 47" chest measurement is comparable to todays size 44.  The suit pants waist will usually be 6" smaller than the jacket size.

If you are seeking a vintage suit with a high waist fit, make sure the front and back rise are longer that those of a natural waist - which average 13" front rise and 18" back rise.  The rise is a measurement from the top of the waistband to the crotch seam or center of the crotch gusset.

Hope this basic guide helps you and Happy Shopping!

If you have any questions on vintage dating or need further help, please feel free to email me thru ASQ or post a query on the Vintage Clothing Discussion Board (Community).