There are several signs that may indicate that your car's exhaust system needs work. These include:

1. Loud noise or rumbling sound coming from under the car.
2. Reduced fuel efficiency.
3. Decreased engine power.
4. Burning smell from the engine.
5. The smell of exhaust fumes inside the car.
6. The check engine light illuminating on the dashboard.

If you notice any of these signs, it may be time to take your car to a mechanic to have the exhaust system checked out.