A trained ear could probably tell just by listening whether or not it has cats. If the mufflers were SLP, magnapacks or stingers I wouldnt be too sure. But a car can only be so loud with cats.
I used to run a bassani catted x. Switching from that to the O/R H, there was only a difference in volume 2500+ rpm. Anything under was basically the same. Just keep the RPMs down and you should be ok. Besides, how often do you hear someone with a mustang fuss about getting a ticket for their exhaust? I've been pulled over 3 times with no cats (not for the exhaust) and they never questioned my exhaust. Probably just the import crowd mostly that gets bothered.
FWIW, I had an MRT o/r h on my 03 Cobra with a Borla and also Mac cb and it was loud. The Borla (straight thru muffler like Magnaflow) was crazy loud at anything above 1/2 throttle so I put the Mac on to quiet it. chambered muffler
my setup sounded great, it was loud, but it wasnt a nasty sound. and i could tell, at idle, they cant, but if they pull you over and they know the smell, they could. but i would not worry about it. But whoever said it is right, above half throttle, it is loud, but it wasnt loud in the car, at least not in mine (but i had a stereo that was up all the time).
I have magnaflows with an o/r bassani x and i haven't had any problems so far, actaully it is fairly quite until u get about 2,500 rpm or are full throttle, as others had said it is not that loud in the car bout have someone else drive around the block and u would be a little surprised how loud the car is. I have been thinking of changing out my magnaflows for either packs, bassani, or borla to make it a little louder. One thing that does give it away that my car doesn't have cats is the smell, instant difference when the pipe was changed.
I am a little worried that in the warmer weather and the police might have their windows down, I will get stuck at a red light and they will smell it. Hopefully they will be using the a/c tho, haha.