How do I know whether I have cirrhosis if I feel well?

Bx or Radiology. Dr hack is correct, with liver biopsy being a common means to diagnosis cirrhosis. However there are findings radiographicaly (ct, us or nuclear) that can suggest cirrhosis in which pathologic confirmation is not necessary.
Why do you ask? If you feel really fine, I'll bet you don't have cirrhosis. If you used to drink heavily but now feel well, I'll bet you're fine. If you're being followed for hepatitis B or C, ask your physician how far it's gotten. If you've got some other reason for asking, ask whether imaging studies are in order. If you've just got some odd labs, you've got a right to a full explanation. Best wishes.
Liver biopsy. Cirrhosis is typically diagnosed on liver biopsy. There are blood and x-ray tests which may also suggest cirrhosis. People typically do well with liver disease until the liver function is severely impaired, so proper medical evaluation is critical.