Determine whether a thread is blocked

Question

Does anyone know of a way to determine whether a thread is currently blocking? Basically, I want to check whether a certain thread is blocking (in this case on a AF_UNIX datagram socket receive call) and send it a signal to interrupt if it is.

I'm working on Linux using Boost.Thread meaning underneath I'm using pthreads. My system has NPTL.

I think the answer is "no", but want to see if I'm missing something.