Here is a good starting point for anyone.
This list, which is posted on the club site, is an extract from TCR of all NV chips known to exist in a cancelled state. Reference to your TCR will show you that some of these are in fact only known cancelled.
http://ccgtcc.marlowcasinochips.com/cancelled.html

You should be wary of any of the following:

1. Any chip listed in the table I referenced which is NOT cancelled.
2. Any uncancelled high $$ item whether listed in the table or not.

By cancelled I mean notched, drilled, overstamped etc.

Here is what you can do if you still have an interest in an item which fits that criteria:

1. Ask the seller for a guarantee (preferably written) that the chip has not been modified.
2. Ask your friendly dentist if he will xray the chip for you to see if there is any sign of alteration.