Do you know what that building is?
The following post includes information on the visual differences that are present in some buildings under construction that can give away its identity, such as in the image above.
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Let's start with the example I used before. Here is a picture of an early Protoss structure. Can you tell if it is a gateway or a forge?
If we compare a forge and a gateway side by side, we can see why forges are easy to spot at a glance. Forges have a lower health bar.
The reason the health bars are placed at different heights is due to the size of the building being constructed. The completed forge is the smallest 3x3 Protoss structure, so its health bar is close to the ground, while a gateway is one of the largest 3x3 Protoss structures.
Also note that while the health bars are at different heights, the position of the warping blue ball remains constant. Notice how the health bar of the forge cuts across the top of the of the blue ball, while the health bar of the gateway rests slightly above.
This idea of health bar height is much clearer if we compare the 2x2 Protoss structures.
Which of the following is a dark shrine?
Dark shrines are particularly noticeable for another reason. While being a 2x2 building, the warp animation is quite different from the pylon and cannon. The dark shrine takes 100 seconds to build. From around seconds 17-95 the dark shrine will remain in the "ball of water" phase, which looks like this
Both a cannon and pylon go through the same process, just at an accelerated rate. All 3 structures take 17 seconds to get to the "ball of water" phase, and at 5 seconds before structure completion, the building will start to shimmer into its final form. So for a pylon, only from seconds 17 - 20 does it look like a dark shrine. For a cannon, its a little longer, seconds 17 - 35. So while you can easily click on a dark shrine to confirm its identity, by just seeing a 2x2 protoss structure warping in for a few seconds, you can easily determine what it is by the speed with which it completes, or more easily, by the height of its health bar.
The final piece of useful information comes with regards to the Protoss tech choices. There are minor differences in the height of the health bars for the Robo, Council and Stargate.
What is far more telling is that the stargate has a distinctive shadow underneath it. This is something that is unique to the stargate and the pylon.
For completeness, here is an image of all protoss structures in a general ordering of height of health bars. Note how the forge and robo are quite small while the stargate and dark shrine are quite a bit higher.
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The zerg buildings give off very little since they all use the same morphing animation. The only difference is the size of the structure and the height of the health bar.
Lets start with one of the more obvious differences, a morphing Hive is much larger than a morphing Lair, in particular the five large "hook" things are quite a bit bigger when morphing to Hive than when morphing to Lair.
The early game structures of the zerg are the spawning pool, evolution chamber, baneling nest and roach warren. Again we can see that the height of each structure is different, most notably the baneling nest is quite a bit higher than the others.
Similar to the dark shrine, the spire is considerably taller than other 2x2 structures, making it easy to spot.
Also notice that the spine crawler is taller than the spore crawler.
Given this information, it should be clear which structures are spires, spores and spines in the following image.
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Terran buildings give away the least amount of information. From what I can tell, a barracks, factory and starport (and most other buildings) use the exact same animation until around 70% through where they change to resemble their final form. The factory's health bar is slightly lower than that of the other two structures but the difference is negligible.
The turret and sensor tower have circular structures. Since the sensor tower is much taller, we can again use the height of the health bar to distinguish between the two at a glance.
And all terran structures, sorted by height.
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In most situations, the player can simply click on the structure to confirm its identity. But in those rare cases when you only get a glimpse, such as a scouting zergling or overlord, then knowing this information could be beneficial.
Also, a very minor detail, but one might prefer to build certain buildings vertically rather than horizontally. For example, building two evolution chambers horizontally makes it easy to compare the height of the health bars and see that they are indeed the same structure. Building them vertically makes this comparison harder. Which in turn makes it easier to build, say a roach warren, underneath an evolution chamber and have an opponent think it may be a second evolution chamber if they didn't get enough time to click on it and confirm before their scouting zergling died.
I also think it's just good to know more about the nuances game too, but maybe that's just me.