I will give my personal experience since I cruised this way for a couple of years. The rules I learned are those:
1. Be sure that the restroom is popular among the gay population. Normally, it should have a discrete location, i.e. not close to an area where a lot of people move around, for examples families. The optimum gay restroom is located in an area with less traffic since it means that the cruising can take place without interruption.
2. Before starting the cruising, keep the restroom under control. Stay outside if you are uncertain and simply watch the traffic. If there is a lot of lone males going there and they stay for a while, i.e. not exiting within a minute after a short pee etc, then it begin to look good for cruising.
3. When you have noticed that there are a guy that seems to take a "long" time and the other two steps has been followed, it is possible to make a move by just entering the place and see what is happening. If the guy that "takes long time" is at the urinals, the probability is obvious that he is cruising. Normally a pee is a quick affair, especially when they are alone. Staying for five minutes at the urinal is an indicator of cruising.
4. In some restrooms, the gay people goes straight to the stalls rather than staying at the urinal. If the restroom is regarded as "safe", they often keep their doors slightly open so other gay men can see what they are doing (jacking off).
5. If those signs are there and the guy stays at the urinal, just go there and put out the cock while seeing what he is doing (to make everything sure). A gay man will certainly stroke it and show that he is interested. It is possible to go to a stall from there on - or even go outside.
Those five rules worked for me when I cruised the restrooms. Some restrooms weren't suitable for cruising and it was demonstrated by the failure of those rules of thumb. A restroom located in the middle of a public park where a lot of traffic passed by has been off the list for example.
I have cruised restrooms in public parks but they have fulfilled the discrete requirement, either by being a bit hidden by bushes and trees or having a location with a particular "lane going to the restroom". That meant that (which I learned from just being outside and study the traffic from above) in 99% of the cases, a lone male that wandered that way was a gay cruiser. In the other 1% of the cases, they simply didn't enter the restroom at all.
In my cases, it was a rewarding cruising time and I got blown many times by hot men/daddies that liked to please me.
The five rules of thumb learned me to find all cruising restrooms in the city where I lived at that time since I could read the "signs" and quickly disregard a place or give it a try.
The police has never bothered me since the places has been discrete enough to not stir up attention from that direction. The idea has been to cruise discretely so "ordinary people" don't notices it. That requirement is fulfilled by discrete locations that aren't obvious to the general public.
I have also learned that waiting outside and watch the traffic is a very good way to see if everything is OK for cruising or not.