1- Inspect vacuum hoses for improper routing or disconnections
2-Look for kinks,tears or cuts
3-Check for vacuum hose routing and wear hot spots, such as the exhaust mani or the EGR tubes.
4-Make sure there is no evidence of oil or transmission fluid in vac hose connections. (Valves can get contaminated by oil getting inside)
5-Inspect vacuum system devices for damage. (Dents in cans;bypass vlaves;broken nipples on vacuum control valves; broken tees in vac lines)

Get a vac gauge and connect it to the intake manifold. Make sure the engine is warm and idling. A good engine should be between 17-22 in. Hg. On some 4 -6 cylinder engines a reading of 15 in HG is acceptable. A slight flicker of the needle is normal with a high performance engine.

Vacuum can also leak from:
1-Faulty intake manifold gasket
2-Cracked intake manifold
3-faulty injector O-rings

Most effective way to test for a vac leak is with a smoke leak detector, they effin rule.

You connect the tester to a throttle bore and then depress the push button to introduce smoke into the intake system. If there is a leak in the system you can see smoke coming from it.

Another way is to use a propane tank with a low outlet pressure line. Make sure there is no source of ignition in or on the engine compartment, Then when the engine running direct a small amount of propane at potential areas of leakage. When the propane is introduced toward the vac leak the engine will pull in the extra fuel and the idle with change. Turn the propane on and off at the site of the leak to confirm your suspicion..