Both from my research. I got my last ticket from this. It's bull but they can get away with it.Now are you talking guesstimate as, they pace you and say you were going 45 mph, based on their speed..or guesstimate as in , you drive past them while they are pulled over, and they guessimate how fast you were going?
I've thought about radar detectors but I have yet to be caught by a cop sitting on the side of the road (yet I've got 6 points on my liscense) so I don't think it would help me. I think the best way to avoid tickets is to learn to see cops before they see you, for the most part it has worked for me but it takes a lot of attention.
I happen to know that around here most speeding tickets are not the result of traditional radars. Speed cameras, radar vans and such take up a good chunk of the tickets and I have yet to see a detector that works with those.