I'll usually go up to the top of a ridge near here and coast down after I've changed tires, just to check on the accuracy. There's a couple of miles of measured mileposts, conveniently placed so I can check while going about as straight as I can. It's easy enough to go in and adjust the wheel diameter in the cycling computer after you know how far off it is; mine are usually within 1%. Start low, end up high, as the tire wears down.
In another sense, it doesn't really matter, because you'll be checking against somebody else's measurements, and you don't know how accurate those are. Some of the AC maps seem to be off half a mile between measurements, and after adding up a day's worth of random errors in either direction, you learn to calculate in your head from one turn to the next, then keep your head up and your eyes open. Similar stuff happens with others' routes in other places; that's why some people will circle the parking lot at the end of a century (to make sure their computer is over 100 miles), and others will complain about how far over 100 miles they ended up riding.
(But it drives me nuts when somebody says the ride I was on tonight is 27 or 28 miles. It's 25.5, to within 1%, because that's what MY measurement says!!)