Even if the engine light is disabled, you should still be able to pull an ECM code to see if there's a rich or lean code floating around in the ECM's memory.
I dont know of any way to really test the O2....since these arent widebands, I dont know if testing them would be effective since theyre basically made to read only drastic differences in a/f mixture....like very rich, or very lean. I've run pig rich before, with brand new O2's, and they didn't throw a code. It takes a lot to make them go off....
It might be possible to CLEAN them, but....I've rarely heard of anyone doing this...
By the way....cheap + anal is a very bad combination. 
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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