There are several signs that your car's intake manifold gasket is failing, including:

1. A decrease in engine performance, such as poor acceleration or stalling.
2. A rough and/or erratic idle.
3. Overheating of the engine.
4. Coolant leaks from the intake manifold.
5. A sweet smell of coolant coming from the engine.

If you suspect that your intake manifold gasket is failing, it is best to get it checked by a qualified mechanic as soon as possible to prevent further damage to your engine.