Apply vacuum to the vacuum tube, either using a vacuum tester, or sucking on the tube. The stem should pull in and not bleed off. There is one exception. Some pull-offs have one or more relief holes making testing difficult. You either have to block all the holes, or while running the engine at idle watch to see if the vacuum holds the stem in. Keep in mind that some carburetor have a primary pull-off that is sometimes used as a secondary pull-off. Always look for a relief hole. Any relief hole should be treated as a secondary pull-off.
How to test a secondary pull-off
