I made a couple of DIY smoke machines and was not satisfied with them so bought one... and it turned out to not be much better. There are videos on how to make one, it is simple and cheap.
You can also use a hand vacuum pump to check the vacuum system, it won't tell you where the leak is but if it holds vacuum you know there is not a leak.
From my limited experience I get the impression the vacuum canister, just in front of the AOS, and the lines around it are often the problem. I have tested 3 canisters and all 3 had cracks around the base and leaked. On one engine the vacuum lines near the canister were brittle and broken, while the other lines were OK. Perhaps they get hotter because they are the lines closest to the engine block.
Some good info here
Vaccum Canister replacement and other vacuum stuff