I usually take pictures of the stuff I map, so I have used some guesstimates to determine heights. I.e. I have a picture of a building im trying to map: 1. It has a door, I assume the door is 2.2m high. 2. Measure the height of the door in cm/pixels, 3. Calculate how many real life meters a cm/pixel on the picture is. 4. Measure the height of the building and muliply it by your value. Now you have an estimate of the building height. If you area has lidar-imagery available (and open) there is normally a point cloud where you can pan and zoom and rotate and look at objects in 3D and measure.

I usually take pictures of the stuff I map, so I have used some guesstimates to determine heights. I.e. I have a picture of a building im trying to map: 1. It has a door, I assume the door is 2.2m high. 2. Measure the height of the door in cm/pixels, 3. Calculate how many real life meters a cm/pixel on the picture is. 4. Measure the height of the building and muliply it by your value. Now you have an estimate of the building height.