I've read a bunch of FAQs and they mention bottoming out, but is there anything you can look for or listen for to clue you in that your car is bottoming out and you need to adjust your springs.
The sparks are often the most obvious method of seeing if you bottom out or not. If you drive in bumper or roof cam, as Duck says, you can't see it, but watch the replay instead and you'll know. You can often feel the vibrations as well.
Not all bottoming out actually slows you down, in fact the game makes sparks just for show sometimes - I speak here from experience at very high speeds, as we get a lot of sparks with no performance penalty in the 300mph Club. In regular circuit driving, though, you're often thrown off-balance by bottoming out.
try all the sparks coming out of your car when it touches the ground!
