A great area for Civil War collecting is uniforms and headgear.  As with other fields, you have to either know what you are doing or need to find a good source, since both areas are loaded with replica examples and can be difficult to discern from the real thing.  Many times old theatrical or re-enactor (living history) clothing items are advertised as "authentic"--and post Civil War items from the Indian Wars are very similar in general looks.  Union uniforms and headgear are the most common original examples that are found since the number of soldiers in the North was far more vast than in the South.  Artillery and cavalry shell jackets were still in surplus as late as WWI in the US and still turn up today in unused condition!  Likewise, there were many forage "bummer" caps left over from the Civil War years.  Early theatrical companies often used AUTHENTIC uniforms in their wardrobes in many movies through the 1950's.

It is not an easy task to tell an authentic uniform from a well-made and worn replica unless you know basics about the types of cloth and thread used in the 1860's.  A key for many is non-mercerized cotton thread as opposed to mercerized cotton or synthetic blends but again, unless you are well educated in the scientific evaluations involved, you need to exercise caution.  The type of broadcloth wool used in officer coats is very different from modern wool and enlisted grade "shoddy" has never been accurately reproduced.  Unfortunately, if you have only seen pictures in a Time-Life book, that will not help you all that much in making an accurate determination.

As a general rule, a quick warning sign on a coat advertised as original Civil War would be the presence of light pin striped material (called "pillow ticking") in the sleeve lining.  That is almost always an indicator of NOT being authentic to the era.  Civil War Union coats are deep navy blue, NOT black, and the inner lining is generally a polished green shiny cotton blend--not black and not brown.  The bottom edge of a long coat generally does NOT have a stitched hem binding--it is called a "raw selvedge"--and is not reinforced.  Civil War coats tend to have a "balloon" to the sleeves which makes the width at the elbow about twice the width at the cuff.  Early coats have functional cuffs, too, meaning they actually will button.  Collar height varies but is generally no more than an inch.  The skirts of a frock coat are very long and extend almost to the knee--short coats are indicative of later examples.  With Confederate coats, most of the same rules apply.  The texture of Confederate wool is generally very coarse to the touch and all officer coats are double breasted.  Rank is affixed to the collar rather than the shoulder, and linings can be tan or brown.  You should also look for a "keyhole" shape to the button holes and there should be ample hand stitching throughout--not many sewing machines were used.  Enlisted Confederate coats are often a very light brown called "butternut", though gray coats do exist.  The gray is generally a steel color rather than off blue or pale cadet blue-gray.

Headgear can be even trickier!  Kepi hats look very similar from 1855 to 1885.  Beware short crowns and non-functional chin straps.  Beware buttons mounted with brass ties rather than sewing thread.  Always check the maker backmarks on the buttons to make sure they are the correct period buttons.  If it has a reinforcement at the back, it is probably post-Civil War.  If the top of the crown is flush rather than recessed, it is probably post-Civil War.  If it is an oval slouch type hat, it generally will have a reinforced border of thin silk and the inner sweat band will be well over 2" tall inside.  There are no size labels in Civil War slouch hats as a rule; forage caps DO have a size marking on the crown if it was made by a contractor and again, you can check a makers guide such as McGuinn and Bazelon to see if the listed street address was used during the Civil War years.  Many hatters (like Bent and Bush of Boston) were in business before AND after the Civil War.  Other makers like Lilley and Pettibone are POST-CIVIL WAR entirely.

A little knowledge will take you a long way but when in doubt, consult an expert.  There are probably more kepi hats around today than there were in 1865--and maybe more uniforms as well.

Rare Union Zouave Uniform grouping, Confederate Officer Kepi and Union enlisted forage "Bummers" cap