usable ecu?

[240marcuSX]

how can i tell if an ecu is usable for my car, off another car? my friend bought a cheap 89 s13, and he'll sell me the head, intake mani, and ecu for super cheap, but is there anyway that i can look at them to see if they might be usable? i know ill have to take the head and mani to a machine shop to see if theyre arent warped or whatnot, but how can i check the ecu? and one more thing, are all s13 ecus compatible? his is an 89 and mine is a 90, thanks.