I've got 85,000 miles on her and still the original O2 sensor. Don't notice any hesitating, idle sometimes is a litle rough, but not dying. When should you replace the O2 sensor? A certain mileage?
Without an O-Scope of some sort you really can't test it. It may be about due though, I think the suggested service interval is around 60,000...but it should throw a code if begins to cycle too slowly for the ECM....right?