How can I tell reliably if a boost thread has exited its run method?

I assumed joinable would indicate this, however, it does not seem to be the case.

In a worker class, I was trying to indicate that it was still processing through a predicate:

bool isRunning(){return thread_->joinable();}

Wouldn't a thread that has exited not be joinable? What am I missing... what is the meaning of boost thread::joinable?