The best way is with a smoke machine. If you don't have access to one, either do a very close visual inspection of ALL vacuum lines, use propane around the intake manifold, or vacuum gauge on all lines. Good luck.
You can use brake cleaner or a can of quick start. Get 2-3 cans and spray all lines with vehicle running. Listen to the idle, when it drops at a certain area you're spaying at, you found to leak. The other way is by sound sometimes, vac leaks are usually noisy.