I give them a casual wave, sometimes a nod. It's fairly easy to tell an enthusiast from an average driver though.
The real enthusiast will have already noticed you anyway
The average driver only knows they're driving in a car, and probably run a 50% chance of knowing thier car is a toyota, and a 30% chance of knowing it's a Supra.
I was once flagged down by a guy in a BMW, he wanted to ask me about my car... he had had the exact same car a few years earlier, and had put 30 grand into it. He was pretty cool.