98 Z71 Check Engine/02 Sensor?

The check engine light has been on for awhile in my 98 Z71 5.0 Vortec. My mechanic told me it's defintely one of the o2 sensors; he tried resetting the computer last time it was in. Is there any way to tell which one it is? Are both of them the same part so that I can just order one? I've heard that they're not that difficult to replace..... Thanks in advance.

What was the code no.?
Remember a code does not say replace the sensor but check it out.
Many things can trip a 02 sensor code?
Is the heater circuit and fuse good to it?
A good scanner will wead out all sensors and show if it is bad.
MT

why don't we try this 1st:
- buy vented fuel tank cap and can of seafoam
- find a major parts store that is close to a chevron gas station
- get your tank to 1/4 low
- go to that parts store and ask them to reset error codes.
- as soon as they have done that, drive to gas station, remove old gas cap, pour seafoam in(funnel is needed), refill, replace gas cap, get her on freeway and cruise for about 30 min at some 60-70 mph.

also, dirty MAF/MAP sensor can throw CE light. different error code though. clean with carb cleaner, reset code, be happy. done this about a month ago, fixed CE light on several cars, including 1500 Z71, with fuel cap.