Depends on how I'm fishing. With plugs, I'm seeing the rod go down, and, usually, telling one of my partners to get his thumb out of his, nevermind, and get the rod.

With a drift rod, I find that most of my strikes I feel almost unconsciously. I don't know what I feel, just that it's time to strike.

With flies on a sinking line, it's, "There he is!" as I feel the yank. On a floating line, a lot of time you can see them coming from a ways away, and its anticipation and trying not to yank the hook from his mouth as he strikes.