A vacuum leak usually results in higher idle speed than normal. This is because air is leaking into the low pressure intake manifold and bypassing the mass airflow (MAF) sensor. The MAF sensor feeds the mass airflow rate to the ECM. However, with an intake air leak, the ECM is calculating fuel injection quantities that are too low for the actual air entering the engine. The engine will run lean if the feed back fuel control system cannot adapt to sufficiently account for the air leak. You may also see fuel system lean codes stored in the ECM.
Vacuum leaks are usually found with a visual inspection. The most common vacuum leak is a disconnected vacuum hose.
On the other hand, if your car is experiencing a lower idle speed than normal and/or stalling, then the problem usually lies with the idle air control valve. It may need cleaning and there is much written here and on the net to help you with that.
Hope this helps,
Tom