No I have never done that nor do I think I ever would.
I did check other people's ads when I was starting to see what they were offering and the pricing etc....and occassionally I will look up other daycares to see if they have pictures and what their set up is like etc....but beyond that I can't imagine why you would need to go 'undercover' for info.
I see other providers at the park and at early years and if I have a question I sometimes will ask them or else I come on the forum to get feedback. Beyond that I am not sure why people would do that.
Sometimes I find that other providers are too nosy with me. They will ask me all about my set up, rates, number of children, MY ADDRESS (WTH?) and how much I paid for certain things, what I serve for lunch and snacks etc. I don't mind sharing the info most of the time but sometimes I do get the REALLY nosy ones like the one who asks me about my house size, address, etc and I know she wants to do a driveby and check me out and look me up. Those are kind of annoying but whatever floats their boat I guess.