One of the problems with spotting is being able to look the part. If one looks like a total nurd and behaves similarly then one will be viewed with suspicion. Recently my wife and I visited a busy private airfield and had total access to all areas - and we received friendly greetings from everyone we met. Maybe it's because we're "oldies", but it may also have been the way we conducted ourselves?
Don't do anything to arouse suspicion and do obey all signs and warnings.. Also, don't hesitate to talk to people. If they find that you are genuinely interested you'll almost certain receive a welcome. It might help to be a woman.... several years ago my wife and I visited White Waltham and were peering through the gloom into a hangar at something (there was a no entry type sign nearby). A friendly pilot appeared, took Ruth by the hand and led her round the hangar whilst she scribbled down everything in sight. He probably thought we were nutcases, but he was polite enough not to say so!