1990 Toyota Corolla troubleshooting; check engine light is

My check engine light has come on and I have no idea where to begin. I've checked the basic radiator fluid, oil, belts. Which all are ok. Would the check engine light come on if my transmission fluid is low?
My financial situation prevents me from going to my mechanic which charges $90.0 per hour just for his time doing the diognostic and mechanical work, the parts of course will be another cost which I do not have. Thank you for you consideration on this matter.