What tool or electronic tool do I need test the o2 sensor to make sure it's still working properly?

 

Oscilloscope? They often get sluggish when they get old, & the signal crosses the stoichiometric value at a much lower frequency than normal. Some DMMs are fast enough to see it crossing from 0 to 1 volts several times per second. Stoic is somewhere around 0.5 volts??