A good profiler should tell you this. If the threads are compute bound, then yes you will use as many cores as you have threads, if you are blocked doing I/O or on contention it will be less than that.
Given that you aren't on Windows, the following doesn't apply, but a decent profiler should still be able to provide a measurement of CPU cycles burned by your process for a give period of time...
If you are on Windows a good free tool to use is the Windows Performance Toolkit (xperf) which is now part of the platform sdk. It will show you the processor cycles burned for each thread or process for a period of time (as opposed to just elapsed times).