How can i know if my tongue is infected?

Pain, swelling...: If you're concerned about infection, you really ought to see a dentist for evaluation. An infected tongue usually has some sort of lesion there (such as a cut/laceration) to introduce the infection and is painful & swollen. Again, a lot of the diagnosis is based on visual appearance as well, so see a dentist to have it checked out.

Look in the mirror: When the tongue is infected, it usually swells up. If the tongue looks asymmetrical or has oozing from it. It may be infected and you should see your dentist or medical doctor.