How to inspect a O2 sensors?

[Ryan Heo]

Hi.

I replaced all the O2 sensors about 4 months ago. Those were all NTKs.After the sensors replaced, the gas mileage was up to 330 miles per 16 gal in the city driving. However these days are 250 miles per 16 gal. in the city and even 320 miles on the highway. So I would like to check the O2 sensor even though it haven't come CEL up.

Thanks.