I had a copper that you couldn't even tell was a coin- looked totally smooth on both sides..whether wet or dry, tried olive oil, etc. Nothing. Then I tried electrolysis for a few minutes. Details emerged- head, stars, date.. not like new of course, but viewable enough to take pics.
You can also try a pencil rubbing. Put it under thin white paper & go over it with a pencil. Sometimes you can see some details or even a number on the date.