Curious as to how one knows its ok to approach a car in a known dogging area?
I have suggested before that couples who are keen to be watched use a red item (cloth/bra/panty) and hang it from their interior mirror (works for daytimes). Then there's the traditional flashed interior light for nightimes.
But if they do not signal in any way, it's very difficult to know whether to approach or not. If you approach, and they were not wanting to be watched, they'll either drive off or you could get a gun up your nose like SDMR suggests. I tend to initially watch from a distance, and let them see that I am probably watching. If they are uncomfortable, they stop or move off, if they're okay with it they will stay. On two occasions I have had the guys wave at me to come in closer. But I have also had a stupid dick spoiling all my watching fun when he just went marching inquisitively straight up to their car.... Flurry of disengagement, and off they went, and everyone loses out!!...
Hence my promotion of "arranged dogging" on this site.