I watched Ask This Old House the other night and they were fixing squeaky floors that were overlain by carpet. To find the joists, he hammered on the floor until it didn't sound hollow. No problem. Then he used a drill bit (he called it a "special tool") that had a very short length of cutting blades to go through the carpet and underlayment to locate the joist. Because the cutting blades were short and he could drive them all the way into the joist, he was able to distinguish between the joist and underlayment.
I am trying to locate the "special tool." What would it be called.
Also, I'm concerned about having the carpet fibers wrap around the drill bit. Any suggestions?