Just do what I do: Go up to them and start watching. If you are ugly like me then they will call you a 'creep' and then drive off.
I don't own a vehicle so I haven't done that in a parking lot but I live in a big apartment complex and see tons of people everyday when I walk to/from work. Lots of kids as well. All making out. Saw a guy fingering his girl one time. Saw two minors literally fucking in the hallway with the girl up against a building....ummm...can't think of the right word. The pillar that holds the whole building up. One of the legs. Guy had her up against that and was fucking her slowly. About a week prior I had been walking home and overheard her parents yelling at her to not see some boy anymore. Fast forward a week and she is literally just fucking him outside where the parents can't see them probably walking inside her home and kisses her Daddy on the lips like a good little girl and everybody thinks she is an angel.
But yea back on topic. I walk up to people doing shit like that and just watch them. They quickly notice and quickly stop and walk off. I've yet to find people that are so into it that they don't stop. Kinda disappointed tbh. Somehow I am a creep for watching but they aren't creeps for doing illegal things in public. I hate normies.