Smoke tester is the best way to check for leaks. Another option if you don't have access to smoke tester is to take a propane torch with a small hose connected and with the gas flowing (don't light the torch
) pass the hose end over various potential leak points. If the idle changes then it is sucking in propane at that point. Can also use a can of starter fluid, but this is even less precise.
There are a lot of things that can cause a vacuum leak, assuming that is the issue.
If you have a vacuum gauge you can check what it when your car is idling and compare to whatever normal value (i think it should be 20 inHg or more, not sure exact value). Someone with a normal running car might be able to test for you.
Vacuum hose leaks: visually inspect all the vacuum lines attached to the intake manifold for damage: cuts, cracks, loose connections.
PCV valve could be malfunctioning
EGR valve could be stuck open
Brake booster: If pumping brakes affects idle then it could be a leak on the brake booster hose or brake booster diaphragm.
gaskets/mating points between throttle body and manifold, intake hoses
I think a faulty EVAP solenoid could also cause a leak.
With an OBD scanner you should be able to check what your fuel trims are, if there is a leak it should force more fuel to even out AFRs, so you will see positive fuel trim. You could also try taking a look at AFRs directly.