You see a light oil stain under the middle of the car, under the transmission

One fender sits way lower than the others

You see a darker oil stain near the front wheels of the car.

If you look at the exhaust from under, you see some fumes coming out of it where they aren't supposed to

After driving a bit, you feel extra hot inside and notice that the cat is glowing orange

Consistent thick white/bluish smoke that does not go away after 3 minutes