what is the best tecnique to find a vac leak? process to follow?

the 1992 500 sl tends to idle arraticaly at times, specialy when the ac is not on. Any ideas as how to find the culprit. I took the air cleaners out and found a dried out tubing but can't find any more suspects. There is a line that I can see in front of the engine by thermostat but can't tell where ir goes.

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Thanks, Juan