99 passat oxygen sensor

Does any one know how to check the output from an oxygen sensor?

Before heading for the dealer, I wanted to check if either sensor was

bad. A replacement sensor is about $125.00 each, internet price. Too

pricey to just replace doing the trial and error method. Its a 99 Passat

20 valve turbo 4 cyl. The check engine light code came up "manufacture

air fuel metering control" It has been running fine till 2 days ago.

It stalled braking for a stop light twice. Any help would be

appreciated.

Thanks, Jerry