Dan,
Contemporary (that is not a hundred years old) proof coins usually have mirror surface of the fields and matt finish on the design. If the coin you have in mind has those, then it is probably a proof. For proof coins their immaculate condition is very important, since even hairlines are well visible on a mirror surface. Try to avoid purchasing proof coins with such damage. Never wipe a proof coin, even with a softest cloth.
I collect coins and tokens which circulated in Africa from 18th century to 2000. I sell about 7000 illustrated world coins from http://www.avscoins.com.
This is true what Andrey says, however if it is damaged in any way it still remains a proof coin. They are manufactured with highly polished dies and also use a higher pressure to produce the perfect coin. Well almost, because you then get what is called a piedfort proof, which again is produced very similar but they a higher pressure again with a normally double thickness planchet.
