How do I know if these were going bad? I've heard that they couldn't be working right, yet not throw an SES code and such. Is this true?

Also, I'd like to clean my throttle body tomorrow, anything i should be cautious about?

Thanks in advance.

 

There was a link write-up detailing how to check your O2s with a multimeter. Basically, you measure the voltage output and make sure that it fluctuates within a certain range when you rev the engine. I think around .35v or something. Someone else might remember the link or is able to search for it.