I have had diarrhea for 8 months which has caused me to feel like I have an anal fissure. How would I know for sure if I have an anal fissure?

Anal Fissure. The only way you can know if you have an anal fissure is to be examined by a doctor. Pain, bleeding, and rectal irritation can all be signs of various rectal problems. An anal fissure can lead to a peri-rectal abcess, so it is important to find out if that is your problem and if so, have it treated.
Also. In addition to dr kelber's answer, you should be evaluated as to why you have had prolonged diarrhea. This needs proper evaluation probably even more so than the fissure. Until the diarrhea is diagnosed and corrected you will have ongoing problems with your bottom.