How do I check ECU codes?

Sorry I’ve been search for a bit and I find all this stuff on codes and what codes mean what but how do I actually check what codes are coming from my ECU? I don’t have a helms manual but I do have a Haynes manual here but I haven’t really seen anything in it about ECU codes.

It should be in the manual, but what year is your car?

teg tips are you friends…