Acetate records are generally recordings made either in-studio or at home, for various purposes.  They can be just run off of the master tape for test purposes (to see what the artist thinks), demos or recorded messages.  This guide will help you identify whether a record is an acetate.

Recording surface.
    The acetate disk's recording surface is very soft, as the records were actually cut on a machine.  The material for this surface is generally an acetate-based material (usually black, but colours vary), hence the name "acetate."  It has a particular musty odour to it, as well.  If there are any matrix numbers or messages engraved in the lead out (blank area round the label), it will be hand etched, and not machine stamped.  However, if there is no label on the record, usually the company that made the actual disk will emboss their name round the centre hole.  These are companies like Audiodisc or EMIdisc.

Labeling.
    The record labels on acetate disks are usually paper, with the record company's (or sometimes, the manufacturing company's) name printed or handwritten on them.  Also, the titles are either handwritten or typewritten.  Sometimes, on home recordings or demo recordings, there is no paper label used.  Instead, information on the record is handwritten in grease pencil, or in the case of cardboard acetates, in ink.

Base materials.
    The base materials are the actual disk materials that the acetate recording surface covers.  The materials could be cardboard, aluminum or glass.  This makes the weight of the disc very very light in comparison with normal records.  With the cardboard disks, the acetate is but a thin layer, and usually clear.  Also, there is usually at least one hole, but sometimes two or three holes, round the perimeter of the centre hole.  This is a post hole, secondary to the centre hole, for holding the record steady while recording.
    It should be noted, however, that there are some acetates that have no post hole.  This is because some companies, like EMI and PYE records in England, used record cutting machines that were equipped with a vacuum suction device to hold the record steady.  However, if the record is an acetate disc, it should meet the other criteria in this guide.

Playing acetate records.
    Since they were actually cut on a machine, and the recording surface is soft, repeated playing decreases the sound quality.  Considering that most of the acetates you will find have been used, there will be a bit of a hiss to them.  This is no fault of the engineering; acetate records were not designed for repeated use.


Fake acetates.
    Acetate records have been faked as well, and these fakes are not easy to identify.  Sometimes, they were cut from the released record for radio or dance hall purposes.  Sometimes, they were cut to actually mislead the buyer.

    First off, condition must be looked at.  For something like a 50s or 60s acetate, there will usually be some wear, and yellowing of the labels.  The paper should LOOK OLD.  Also, there may be something like the acetate material chipping off round the edge of the record, and the record should play with some hiss, as stated before.  The chipping usually happens when the recording surface dries out, and the base material can be seen underneath.
    Secondly, the label's printing must be taken into account.  Is there a record company name printed or written on the label?  Are there misspellings of the artist's name, or mark-outs with a pen?  These must be checked.  Another thing, is sometimes something will be written on the label in parentheses, like (Flat) or (EQ).  This identifies whether the recording was equalised, or just run immediately off of the master to get an idea of what it would sound like.
    Finally, does the record match the general description of an acetate record, as mentioned in this guide?  The post hole is important for the majority of acetates, because anyone could put a label over a regular released record.  Acetate discs are very light in comparison to released records.  If you have a question about it, and see no post hole, you may want to ask the seller to hold it up to the light to see if there is a hole beneath the label.  In person, you would be able to thump the label area with your finger, and see if lets off a bit of a "ting" sound.

Rarity of acetates.
    Some acetate records have different mixes of a recording, rather than the released version.  If you have a commercially-issued version of the recording on hand, you may want to compare the two records to see how they differ.  The alternate version may be one-of-a-kind, and the value is much higher than simply a released version on an acetate record. 
    Acetates are rare enough anyway, though, as there are usually ten or fewer copies made of them.  This does not take into account things like Fonotone records, which is an actual record label that released all of its records on acetates.  They are still rare, and command a high price, but there may have been up to a hundred copies made of some of their titles.

Hopefully, this will help you decide if you have an acetate record, or are looking at one.  Some sellers are selling some records as acetates, when they are not.  For example, commercially issued 78s are usually heavy in weight, and made from shellac.  Commercially issued 45s and LPs are usually made of a vinyl composite, or even styrene in some cases.  These materials are the same throughout the disk; there is no difference between the recording surface and the base material.  Also, commercially issued records are pressed, though some may have hand-etched matrix numbers in the lead-out area around the label.