How to tell if I will I bottom out a car on my driveway?

Ask the salesman what the road clearance is on the model you are looking to buy.

Is 3 1/2 inches of ground clearance even legal the road isn't flat like a race track you'll spend lots of time fixing things that get ripped off underneath Ive wasted lots of Sundays welding up exhausts I ripped off my stupidly lowered cars that didn't handle any better than factory height in reality

probably, with lots of math. but if you wanna guess based on minimal effort

you could try just driving your car onto the edge, then getting out and measuring the amount of clearance between the bottom of your car and the pavement. then back up a little bit, remeasure, then go forward, remeasure, try to find the Smallest distance you can record between the bottom of your car and the pavement. lets pretend the smallest distance was 3"

then put your car on a level ground, measure the distance from the ground to the bottom of your car (let's pretend its 10")

we can now assume that the edge to your driveway will take away up to 7" of clearance between the bottom of the car and the pavement.

so in this case (if the wheel size/wheelbase lengths are similar), if the bottom of the new car was less than 7" from the ground, you wouldnt have enough clearance to get up without scraping, unless you drove it up from a sideways angle instead of a head-on angle (and even that might not work)