If the new cat fails because a fault develops after you fit it how can you possibly guarantee it?

All you can do, as you say, is check the gases after fitting, check that you have Lambda around 1.00 and check that fuel trims are looking sensible. Also check EOBD Mode 7 to look for pending DTCs - any stored here may indicate a fault brewing.

Print all the evidence and file it with the vehicle records in case the worst happens.

Unless you have a crystal ball that's the best anyone can expect you to do...

Regards,
Steve