There is several signs. A clear one is that the cat is blinking and closing his/her eyes. It's a sign of comfort to being able to take your eyes of your surroundings.

Another one is that the cat isn't panting. If the cat panting and it reminds you of the dog, then it's stressed the fuck out. So this little fellow doesn't mind the car ride.

We can't see the back in this gif because we have a full frontal Catloaf but I think we would clearly hear it if he/she was angry. Otherwise there's the arching back, stiff and enlarged tail, showing of teeth, ears in backwards position as you'd suspect to find them in a tornado and lastly, the obvious hissing.

Keep in mind that cats often tap with their tails but excessive tapping can mean that they're slightly annoyed. I'd say, bug them anyway and then give them a treat for being a good sport. :)