how can u tell?

 

you hear it bogging out and u feel the car jocking a little bit..

 

If it is a slight misfire, you may not feel much at all. It just feels kind of like you are getting on and off the gas really fast. The worse/more it's misfiring, it will feel like you are letting off/getting on the gas more and more. Also, the faster the engine is spinning, probably the smaller amount of time between pulses...

 

when the check engine light says so.

 

Depends on how bad it is misfiring and what is causing it. A bad O2 sensor can cause to car to misfire because it will run way too rich. In this case, the car idles slightly rougher, and you can hear a "putt___putt_putt_________putt___putt" kinda sound from the muffler while the car is idling. However, other causes of misfire may give you a misfire at certain RPM's or certain load, etc. The ECU should detect it eventually and give you a code.

What are your car's symptoms?

 

Misfire will not always illuminate the CEL