Late in the evening I create an agenda for tomorrow's day. I usually have a good idea of how much free time I'll have, and how I should spend it. If the next day I didn't accomplish what I wanted, I'll look for a reason. Sometimes it's because something urgent came up; sometimes it's because I actually didn't have to do the tasks I thought I did; but most of the time it's because I didn't use my time efficiently. That's procrastination: follow luke's advice. (Note: it's rare when I didn't get everything done because I put too much on my agenda, but if that happens, turn it down the next time.)
In the moment, I feel procrastination when I'm not looking at my computer screen (or whatever object I'm are working with), when my focus is somewhere else. I think most people call it spacing out. It takes me about a minute or two to catch myself doing that. As soon as I do, I guide my attention back to the computer. (Usually this happens right after I finish doing something, and have not yet decided on my next step.)
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.