Unless you have a precat and post cat o2 sensor with scan tool hooked to computer, you dont, so as far as I know you will need a gas sniffer. The cat can make that smell from a bad state of tune or bad fuel. Antifreeze makes a funky smell too, as it destroys the cat. Are you due a inspection or something? You can do a back pressure test if you feel like it is getting blocked up. Really no way to check for proper operation without measuring input and output gases though.