If it's slick and not visible, is that it? Or are there any tricks you use to try to determine the date? Special lighting? Magnification? Other markings elsewhere on the coin? EDIT: On silver coins
Depends on all sorts of things. Sometimes, oblique lighting (exaggerating relief on all the coin's features) will help. Sometimes magnification helps. For many series, you can look for die markers and such to identify some dates. (For example, it's not hard to tell whether a very slick Type I Standing Liberty quarter is 1916 or 1917, even if the date is completely gone.) For cupronickel coins, you can sometimes etch them to restore the date, although you're left with a damaged coin. For silver, a few people swear it's sometimes possible to restore the date, but results are spotty. For some coins, no, there isn't anything you can do.
Thanks for the tips. Different lighting and magnification did not work. I don't want to try acid or other methods that would decrease the silver content of the coin. What's frustrating is it seems like I can begin to see something with shadows but it's probably just my imagination...
Several possibilities - First - If slick and not crusty or corroded try placing a piece of white paper over it and rubbing with charcoal. If you start seeing something move down to a thinner paper and try again. Second - put a piece of foil over it and try to create a reverse image of what's there. Third - exactly the same but using silly putty. Last option. Ink it with a roller. Place a piece of paper on it and leave until dry then try to read. You will probably smear the image if you try to remove the paper right away. If you don't like to use ink candle smoke can also be used but you will probably burn your fingers. Good luck.
Certain coins have special die or design characteristics which can be attributed to certain specific dates or range of dates. So specifics and pictures always help rather than a generic question and a generic reply.