If it's an actual ring on the disc, press your finger against it lightly. If you can feel it right away, the disc is forgeted, if it feels smooth or very nearly smooth take it to somewhere that deals with damaged discs if you really care that much. An actual scratch on the disc isn't something you'll fix at home with stuff you're likely to have, and if it's deep the actual data on the disc is probably disrupted. (Even if it's not deep the data is still probably messed up, and it'll freeze and randomly forget up if it runs at all)