You can pretty easily tell by listening to the op calling CQ what sort of response makes the most sense, whether in a contest of not.
Even on a Tuesday, if I hear someone sending short CQs and only sending 5NN to the station he answers, I'm pretty sure he isn't interested in ragchewing with me.
Even on a Saturday, if I hear someone sending 3x3 CQs I'm pretty sure he isn't interested in me sending him just 5NN MD.
In between those extremes, if you hear an obvious rate junkie calling CQ TEST at 40 wpm and don't want to just send them minimum, keep spinning the VFO knob. There are ways good contesters can signal they would rather work you than have you do that - On CW they will be sending more slowly, on phone they might throw in "Anyone, anywhere" etc.
Fred K3ZO, a recent SK, was a Hall of Fame contester that never once complained about the "Please copy..." and "QSL 599 Maryland, your..." and never sent CW all that fast and always acknowledged a "Hi, Fred" - but was a perennial top 10 finisher in phone and CW contests. Fred also NEVER sent an extraneous character or word when he was working a rate junkie!
73 John K3TN