As others already mentioned, the length of the head tube gives a good clue on the frame size.

I also pay attention to the location of a pump peg (if present of course) and the distance from the peg to the top and the down tubes.

If the distance between the peg and down tube is roughly double of the distance between the peg and the top tube then that's a good indication of a 56cm frame:




If the distances from the peg to the top tube and the bottom tube are about the same - its 54 cm. Important note here: the distance should be roughly equal to the top tube to peg distance on 56cm frame (about 1"). On 52 cm frames the distances from the peg to top/down tubes are about the same too but they are much smaller (about 1/2").
Example of 54 cm frame:


if the peg to down tube distance is about 2.5-3 times greater than the peg to top tube distance then it could be 57-58 cm frame. But it's much harder to eyeball the frames that are larger than 56cm (in my experience). Here's an example of a 57 cm frame:



As far as I remember, the 50cm frames and smaller will not have the pump peg.

Hope this helps.

And yes, I am showing off my bikes ... cause i love them and they are pretty