There is no way to tell on your own. The only way to be certain is to see a doctor. Mono is very dangerous, and can be fatal, so it is best to get tested if you even suspect that you might be infected.
Mononucleosis, a viral illness has typical symptoms, including: sore throat, swollen lymph nodes, EXTREME fatigue, and malaise. To verify diagnosis, blood tests (titers and Monospot) are completed.