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-offSome pull-offs have relief holes that make testing difficult. Here is an example: The steel cap with the 3 holes is covering a filter that is positioned over 1 small bleed hole in the top. The hole is designed small enough that with normal vacuum from a car it will pull up and hold while running and release once the car is turned off. It can be difficult to test these with hand pump while trying to seal off the bleed hole. We use a electric vacuum pump that pulls about 20 inches of vacuum on it and we then block off the bleed hole for testing for leaks.