How can I tell if my o2 sensors are working properly?

Don't worry about #2. It'* just a catalytic sniffer. Hard to tell if an O2 is bad. Most obvious problem is poor mileage, usually at over 50-60k on the sensor. If it'* been that long, change it.

Yeah if you think it might be the 02 you might as well change it becuase you car is due for one anyways

I had really poor mileage, about 230-240miles to the tank. It was never that good to begin with but after I replaced it I got about 30 and sometimes 40 more miles to the tank. It'* pretty cheap from http://www.gmpartsdirect.com, I think $42 but it'* been a while since I got it. Very easy to change.

Napa is the cheapest place I found for one. They want 70$. (Keep in mind, this is in Edmonton). I don't really want to change this out unless I need to , for that price. I bought the car 3 months ago, so I don't know when or if the sensor has been changed.

so does that mean mines shot if i only get like 250 a tank? on my 92 ssei...and thats with me not beating on it...i think im at like 180 right now and i have a little over 1/4 tank...

Forgot you're in Canada

Do you have any info on the previous owner?

Sounds like your mileage is pretty similar to mine sometimes. Do you do city driving mostly or highway? Sounds like it could be a problem. It wouldn't hurt to get a new one. But get the ACDelco and not the Bosch. There'* too many bad accounts of using Bosch.
I changed my fuel filter, PCV valve, and O2 sensor at the same time, which also helped my mileage.

combination on milage...wahts a pcv valve...

Honestly, I don't have much of a clue. I just know where it is and how to change it. It costs less than $3 and you can change it in less than 3 minutes. And if it rattles when you shake it, it should be good. Mine rattled but I changed it anyway. But you have the */C so yours may be in a different location than mine. I'll check into it and let you know where it is unless someone else answers first.