The engine doesn't start up very easily and will often not start at all. When it is running it can cut out completely without warning.
The engine sounds like it's not running correctly, the engine sounds rough as if it's not running in time.
The fuel gauge seems to be going down quicker than usual, the car is using more fuel when driving in the same way as before.
The dashboard warning light showing on this page (or one very similar, sometimes a slightly different shape or colour) has appeared on the dashboard and doesn't go off when the car is running.
When the engine is running it does sound like it would normally, it can sound like it's struggling to keep running or produce strange noises that sound like it's not running in time with itself.