Lynn, technically it is the lack of P waves on an EKG.
I have had a lot of experience with intermitent afib over the past 6 years or so. It comes and goes, but mostly has been under control with a good med, no caffine and supplements.
Afib occurs when the upper chamber (the atria) of your heart starts fibrillating rather beating normally. I picture it as sort of a quivering motion. The electrical impulses that should control it get stuck in a re-entrant pattern so that you don't get good solid beats.
The lower chamber (ventricle) tries to compensate for the decrease in flow by beating harder and faster.
Physically, the symptoms I feel are a very rapid, irregular heart beat. The irregularity is the key. You might get a couple of good, hard beats and then sort of a fluttery feeling if you take your pulse. My rate can go as high as 200, but it usually hangs around 130. Usually it stops on it's own.
If you suspect afib, and EKG is the only sure way to know. But most of us can tell the difference between a strong, steady regular heart beat and one that is unusually fast and irregular.
Hope that helps.