I failed the emission test and hence these questions (yesterday I had posted for that). I read quite a lot of old posts for this . One issue many have noted is o2 sesnsor esp for high HC and CO. There are method of testing it but still the caveat is that a slow o2 sesnsor may give good readings but car can still be running rich because of slow response from it.
Reference:
http://www.brickboard.com/RWD/index.htm?id=618018&show_all=1
So here are the questions:

What is the definative test that tells that the car is running rich, a single symptom which can not be wrong.

Is it by inspection of spark plugs . How do they look when conditions are perfect.(I have haynes which has pictures of all types). Do they have bluish tinge with no carbon deposists whatsoever. I actually never saw them in such condition . Last time (2-3 months back)when I check them there was a thin layer of carbon deposits otherwise they look pretty neat.

I will change these irrespective of their present condition with NGK BPR6ES as they are two years old.

I am asking these questions for O2 sensor? So if they have carbon deposists does the O2 sensor has issues ?
Regards
Gopesh