[officialtony]

I watch the hands in the tie up. If they are positioned on the shoulders of the opponent, then he is blocking off - stalling.
I watch the head in the tie up. If it is forhead to forehead or wedged against the shoulder or chest - stalling.
I watch the forearms/elbows in the tie up. If they are positioned against the opponents biceps or shoulders - stalling.
When mat wrestling:

I watch the hips. If top man stays hip to hip, I don't care if he is trying to put a half in or not. He has about 5 to 8 seconds - otherwise stalling;
If the top man hooks the ankle, he has 5- 8 seconds to attempt to turn and get off the ankle - otherwise stalling;
I watch the off side arm. If it is around the waist and not holding onto an opponents arm or wrist attempting to turn his opponent - stalling;
If the top man does not attempt to break his opponent down or throw a half or some pinning combination in 10 seconds - stalling.

Now, having said all that, I give two verbal warnings ( which I have discussed in my prematch with both teams ). " Work to score " or " work to improve ". Once I give those two commands, and nothing significant happens, someone gets a stalling call. That's how I explain it in the prematch and that is how I enforce it ON the mat.
Hope that helps.