@ Thegentlebull - yes, dogging does happen from time to time. There are unfortunately very few ladies who are involved, and you are most unlikely to come across single ladies brave enough to announce to everyone that they are interested (although there ARE some out there!). More likely it will be a couple with an adventurous lady and a big tough hubby!!!.. Gents meeting up in a carpark for a mutual m2m wank session, in my view, is not dogging!...
There is no "universal" sign that is used by folks who are out there for dogging, although the use of red items on the dashboard or interior mirror, sitting around with park-lights left on, door open and wanking, interior light flashed and left on (at night), etc. seem to be the most frequently used means of recognition. Many different folks have made lots of different suggestions here on this site over the years. Search the threads...
How do you know when it is dogging, and when the couple is just there for a "skelmpie"? If they are obviously uncomfortable with the presence of another person nearby (they shut up shop and move away, they point a finger or a gun at you, they show you the bird as they drive off in a huff...!) then you know that they weren't there for dogging, and just wanted some private space to have their fun. When it is dogging, there is usually some kind of "come on over" indication after a period of checking each other out, either a hand gesture, door strategically opened, headlights flicked, interior light flicked and left on, etc, showing you that you are welcome to join them. But just barging over and saying "I'm here for the dogging" (like I did once!) is a sure-fire way to put a stop to anything happening....
If the couple seem interested, and didn't drive off immediately when you arrived and parked nearby (NOT TOO nearby!), you might want to open your door and wank and let them see that you are busy (kinda like advertising your wares!)..., and hopefully they would invite you over to watch / join them. Otherwise, sometimes you have to be brave enough to take a gentle stroll over towards them (this might also chase them away, of course!), and ask for a light or the time of day, and have they ever heard of Swinging Heaven?....