Here's my 2 cents.
On his pilots license it should say Airline Transport Pilot, meaning he holds the highest level of pilots license one can get, and is likley not CFIing just to build time, and most likely has flown outside of CFIing, and has "real world" experince.
On his CFI ticket the DOT circle on the top right should be gold, signifying the CFI is a gold seal, 80% or higher first time pass rate, with more then a few students under his belt, he also holds a ground instructor rating.
This isn't the end all be all, quite a few subjective and personality type aspects too (see subjective), but what I laid out above is probably the easiest look and verify "black & white" way of doing it.