Quote - How do I tell that it is using the 64-bit version with multiple threads ...

You can see the number of threads by opening the task manager, choosing the "Processes" tab, selecting View | "Select Columns..." and ticking the "Thread Count" box.

Quote - ... does it make a difference if I set my threads to 4 if I only have 2 CPUs?

You'll still get some benefit from the extra threads, but you should compare render times for 2, 3, and 4 threads to see how your machine copes.