Vacuum Leak

There are several ways. First, get yourself a vacuum gauge, and verify that the engine vacuum is good during low idle, when accelerating (just open the throttle while standing still), etc.

Another way is, if you suspect an intake manifold leak, put some soapy water around the base of the manifold (both upper and lower), and watch for bubbles.
Also, check the PCV hoses (my car has three of them, one of which is not visible with the naked eye - you just have to know it's there. I had to take off the upper plenum just to see it, but it was the problem).

If you still can't find a vacuum leak by sight, use a stethoscope.