if you are not getting any error codes that indicate that there is a problem with the oxygen sensor,
Not true, If the ECU throughs a code one, that would indicate an oxygen sensor problem.
how can you tell if you need to change one due to high mileage?
If your engine is running good, there should be no reason to change your oxygen sensor dew to high miles. It doesnt hurt to change them but not nessaury. The way to tell is if one, your ecu is throughing a code, two, your engine preforemce isnt what it use to be. Its normal a noticalbe change.
are the aftermarket sensors as good as the honda ones?
They are. I put a Busch in my old car and it ran fine. Ran me about 50 bucks.