I have heard of numerous DIY methods to detect those pesky vacuum leaks but it usually involves the use of a propane connected to a hose or carburetor spray cleaner aimed at suspected leak sites while the engine is running. Both of these methods can present a nasty fire hazzard when used around a hot engine. Smoke machines are a better method but the cost of these machines is way beyond the budget of your typical home mechanic.
Here's an el-cheapo method I have used successfully. It's based on the smoke generator concept.
1. Acquire a smoke generator - one cigar
2. Remove the PCV valve from the valve cover and plug the hole in the valve cover.
3. Leave the PCV valve connected to the hose that leads to the intake manifold.
4. Attach a long rubber hose to the open end of the PCV valve (the end that is usually screwed into the valve cover).
5. Lite up the cigar and start blowing smoke into the intake manifold via your hose connected to the PCV valve.
Keep blowing smoke through the hose connected to the PCV valve. The spring-loaded anti blowback valve inside the PCV valve will keep your increasing air pressure and smoke inside the manifold. If there are no leaks you should feel a significant amount of pressure build-up and NO smoke. If there is a vacuum leak the air pressure and smoke will escape through it and smoke will be visible.
6.Why did I suggest using a long hose attached to the PCV valve? Because not all vacuum leaks can be detected from the top side of the engine. You and your cigar may have to crawl under the car to observe any smoke that might be leaking out of the lower part of the manifold or its runners.
7. Give it a try - it worked for me. It's safer than propane and carburetor spray and a cheap cigar costs only about $1.50.