You can use an IR temp reader to check temp differential front-rear of the cat.
If it's free-flowing, there shouldn't be much difference. If it's blocked, rear will be significantly cooler.
If it's a pre and post O2 sensor car (ludes aren't so this comment is effectively just wasting the internet - sorry!) you can pop the sensors out and put a pressure gauge in their place to check the pressure either side - same as temp, shouldn't be enormous difference unless blocked.