Not to be rude, but if you read up on what rapid rise is your question will answer itself. If it's a replacement, just find out what the old one is. If you understand how rapid rise works, it's pretty easy to tell just by playing with the RD (though they usually say it right on them). If you're building a new bike, well, do you want to run rapid rise or not?
Edit, just saw you have a pic of the bike in your sig. Like Tim said, yank on the cable there at the seat stay. If the RD wants to move toward the wheel, that's a regular RD. If it wants to move out, that's rapid rise.
Last edited by Landgolier; 10-22-06 at 02:49 PM.