For those who spot at KMSP: I am curious to know if Airport Police at MSP are ok with people standing on the Lindbergh parking garages to watch aircraft. Is it “legal” or do I need to go to the Post Road lot to watch? Thanks.
I don’t know about your location specifically, but I would guess you’re either trespassing or you’re not. If it’s a public parking garage, who’s to say it’s not okay to stop and look up for a while before getting in your car. 
Locally here, we had a public parking lot specifically designated for runway watching but they scrapped it to make a longer runway lol. No where close to watch now. 
I spot at MSP there a lot too, and I’m sure that airport police are fine with it as long as you are a paying parking garage customer. I’ve never had a problem.
I’m not familliar with the the set-up at MSP, but if you are not in a restricted area, I’d guess you’re free to watch away. Given the current aviation security climate, you might expect some quesitons from law enforcement at some point. I was in Texas watching jets and taking pictures one afternoon and I was approached by security. They asked a few questions and then asked if I would come in for further questioning by an investigator (these guys were just walking the beat so to speak). I said I would be happy to. I answered a few questions and let them look at the pictures I took and I was on my way in about 5 mins. All parties involved were respectfull and friendly. It was obvious to me they were doing what I pay taxes for them to do and it was obvious to them I was not up to something. That is how it should work. As a pilot I am pleased to know they are out there doing thier job.
It’s not that easy here in the NYC area, especially EWR, unfortunately. You are a potential terrorist until proven otherwise, and even if you are on public property, there is a very good chance you won’t be standing there for long. The laws and what the police actually practice can be very different at times.