When to replace car fluids?

Thanks for your help. it looks like the car manual needs to be read first. but cars so far I didn't have to know much, now I don't want to stop in the middle of the road because of some maintenance matter that I didn't know. My car has only 64 K miles, don't drive much therefore even oil change is less frequent but this is the only thing I knew I have to watch for; recently I learned about cooling liquid ... now thinking how do I know if gear box, engine oil is dirty? Is there anyway I can check myself? (simple as windshield washer level:)