Low pressure propane or butane directed at the joints with a long small diameter tube is the best way, but WD-40 will do in a pinch. The trick is to isolate the air cleaner so that whatever spray you use can't be sucked into the air cleaner, which will give you a false result. Btw, check the easy stuff first. On top of the manifold/TB is a small metal tube that should have a rubber cap on it unless you have a California emissions system bike. The rubber caps sometimes go missing, like after a backfire through the intake for instance. If the cap is missing you will have a constant vacuum leak.
Jerry