You can swap the two front sensors and see if the code follows the sensor. If it doesn't, look elsewhere.
To check the sensor more carefully someone receintly posted how you can bench test an oxygen sensor using a propane torch and a DVOM. You can find that using a search. It has only been about a week or two.
Normally you check out an oxygen sensor by reading the sensor with the engine running. The voltage should be changing constantly between rich and lean. You can drive the mixture lean by causing a vacuum leak and confirm that the sensor reads lean. You can drive the mixture rich by artifical enrichment with an unlit propane torch feeding propane into the intake and confirm that the sensor reads rich. A lazy sensor, one that is not switching very rapidly, can be from a defect or it could be from not being properly warmed up. If an oxygen sensor remains lazy after warming it up and exercising it from driving the system lean and rich, it probably needs to be replaced.
