I feel tension.
When I am not procrastinating, I am focused on what I am doing. Whan I am procrastinating, there is usually the feeling that something is wrong, which makes me tense -- sometimes the feeling goes aways in a middle of the alternative action, but it is there at the beginning, and it will return when the action is over.
I'm going to steal Anna's idea and change it to the instrumental side of rationality. In Luke's algorithm for beating procrastination, Step 1 is to Notice You Are Procrastinating. I'm not so sure this is easy. For me, the knowledge sort of fades in and out without being explicitly grabbed by my consciousness. If I actually held onto that fact, the moment that I was evading a task, and made it clear to myself that I was doing the sub-optimal, and the consequences involved, I think it would go a long way towards getting me to actually get things done.
What do you use to catch it? How do you notice you're procrastinating? Leave your ideas below (one idea per comment), and upvote the comments that you either: (a) use; or (b) will now try using.