Check Engine Light!!! 91 Explorer

how do i trace down whats causeing the check engine light to come one it doesnt come on all the time just when u get above 25 mph and it comes on and off ity never just stays on and at idle its not on can someone tell me how to test or figure out the problem

head to autozone and have them scan it for free they will tell you what is wrong come back with codes someone on here will tell what they mean go there even if that light is not because when they scan it should trigger it.