It does get a little confusing at times. I regularly fly past one such airfield (Coventry) that sometimes has radar available, sometimes not.
Sometimes I wish a radar info service, sometimes in better weather I am happy with a flight info.
I get round the problem by calling just the airfield name on Approach freq. and waiting for the ATC reply, suffixed with "approach" or "radar" before making the appropriate request.
Sometimes, if it's obviously very busy for the "radar" controller we just ask for flight info anyway to help ease his workload a bit.