Diagnosing a bad cat

Looking for confirmation from those who know.
In the quest to get my truck to pass the Calif emissions test I learned something from a "test only" smog tech yesterday that I thought I'd pass on.
I was told that one way to tell if a cat is bad is by the 02 numbers, if the 02 emisions read's anything over 0.1 your cat is bad or "going bad".
The smog tech as well as a mechanic with a shop next door both explained that basicly a cat converter uses 02 to scrub other emissions so if you have high hydrocarbons "as in my case" and high 02 numbers "above 0.1" the cat is worn and not able to utilize the 02 in the exhaust and flows out to be read by the tester.
I cannot confirm this since I have not changed my cat yet.
A different shop "not a test only" ran my emissions test and offered to diagnose my problem for $85 an hour, 1 hour minimum.
I got fed up and took my truck to the "test only" tech who gave me the info no charge and told me that any tech that half knows what they are doing should be able to diagnose my truck by the test result alone since my 02 emissions are at 0.9 with a high hydrocarbon reading, I'm all for a guy making a buck but damn $85 to tell me what the test numbers show,
I guess I have found a new smog tech and will only go to "test only" shops since they have nothing to gain by a fail and instead rely on repeat business and word of mouth customers
One other thing I learned was that "high flow after market cat's" dont last as long as OEM ones, the tech explained that there is less of the material "catalist" in the after market units and it wears much quicker, my catco lasted 10 yrs and about 100k, I only changed my OEM at 140k as part of installing a better flowing exhaust.
If anyone could confirm or deny what this tech told me I'd really like to hear it since a new cat is going to cost me $300 or so.
Thanks.

are you talking about the oxygen sensor being 0.1 volts? or what? good oxygen sensors before the cat go from .200 - .900 volts. The ones after should be more stable... like .300 - .600 or so. i guess I do not understand the whole thing about it being bad because it is higher than .1?

Confused also. P A B makes valid points.

Another way to check cat efficiency is to get the cat fully warmed up in a drive cycle, then check the inlet temp vs the outlet temp with an infrared thermometer, the outlet should at least 100 degrees more than the inlet if the cat is working.