Test Method #1

Vacuum leaks are common and detract from high performance. The AFM (Air Flow Meter) measures the air being used by the engine. If there is a vacuum leak, the AFM output is in error and a lean mixture will result. This is especially important at idle and low rpms. A hesitation is often the result. If your engine uses an O2 sensor, the O2 sensor can hide symptoms.

In a non FI (Fuel Injected) car, like my VW Bug, I would spray around the intake with carburetor cleaner (STP) or a propane torch (don't light it!). Any vacuum leak will draw fuel into the intake and the idle will go up (rarely it would go down). This is how you know when you have found a vacuum leak. Test this with a shot of carburetor cleaner into the intake of the air filter. The engine RPMs will rise for a moment.

However, on an early 944, you have an O2 sensor that keeps the fuel mixture the same. This will defeat the test above. Disconnect the O2 sensor at the connector.

On late 944s, you have an added idle stabilizer that keeps the idle at the same value. This system too must be turned off before fishing around for vacuum leaks with a can of carburetor cleaner. You have to short B & C on the test connector to disable the idle stabilizer. This too will defeat the above test since the idle would never change if the idle stabilizer is functional.

CAUTION: Using combustibles around a running engine. Keep a fire extinguisher handy at all times. Avoid spraying the spark plugs. Do not spray excessively.

Summary

Before looking for vacuum leaks

Test Method #2

Use a non flammable liquid such as water.  The water will block the air leak and the engine RPMs should rise.