Check engine light on a 95 protege

[AvivaSimplex]

My wife has a 95 Mazda Protege with 65k miles. Despite the relatively low mileage, it's a piece of crap that we want to get rid of.

Recently the check engine light came on. The car continues to run as well as it did before. I'm 75% sure it's the oxygen sensors or emission control devices that's causing the light. If that's the case, we'd probably not fix it. But, could it be a semi-clogged fuel filter or something else important? The maintenance records are spotty, and I'm not sure if the fuel filter has ever been changed.

I don't want to go to a dealership and pay $100 just for them to hook it up to the computer and read out the code. Is there somewhere else I can go to get a readout, or is there another way I can tell what's wrong?