Some people claim to be able to hear it. I find that difficult with road noise, loud intake and exhausts etc. Your check engine light will come on if there is significantly harmful knocking/pinging occurring in your engine. There is an excellent device called 'Knocklink' which uses a bosch donut sensor (essentially a microphone) which is fitted to your block, connected to a series of green/red LEDs that will light up according to knock levels. Effectively used by many many modified EVO/WRX owners, highly recommend it if your car is modified.