If an opponent protoss warps a building, all buildings look the same, or is there a hint, what kind of building it will be once it is finished?
They are all different, either differing in size or shadow. Someone posted a good guide to this on /r/starcraft years ago:
Stargate (has shadow), Twilight Council/Gateway, Robotics Facility/Forge. I would recommend you just play a game against a Protoss friend, get him to make a lot of money and build every building in a line, scout them before they finish, take a screenshot and compare.
Obviously this only when you don't get time to click on the building when you scout, if you only get a glimpse and you need to know if it's a gate or a robo, or a pylon or a dark shrine (that distinction is more obvious).
In most cases you can get a good read for the buildings in production by looking at the width and height of their life bars (under "Gameplay" you have first to activate the life bars – at least for "Damaged"). For example: The life bar of a pylon is seen slightly on top of the warp-circle, the life bar of a cannon is positioned a little bit lower (just inside the circle) and the bar of a dark shrine is positioned way up. Same for the robotics Facility (low), gateway (middle) and starport (up). You can use this also for Zerg buildings.