Well, this is not a science, but I'll give you some guidelines:
You may take pictures inside churches as long as there is not a mass in progress. In certain churches flashes might not be allowed (for art conservation).
Do not take pictures of policemen or certain sensible areas (army buildings, police stations,...). You could make someone ask you why are you taking pictures, and that can be an embarrasing situation being a tourist.
As for people, as long as you don't put your camera at one inch you would probably have no problem
. But if you doubt, ask first
Fernando