You can tell if people slow down and watch you. Back when I was fast, I would sometimes run the last mile of my 12-14 milers all out. At my fastest, I did 4:39 for the last mile through the suburban neighborhood (three times over the course of a couple of weeks). Each time cars driving by would slow down and watch out their windows or using their mirrors and kind of pace me for a relatively long time.
I was also getting people in passing cars to slow down and watchs a couple years ago when I was probably hitting 4:30-5:00 pace on sections of the downhill running down a 5 mile long gravel road with 2,000 ft. drop (after running the uphill to get there).