@Christopher Beaven nailed it. I always look at the feet, and which way the clothing or whatever is turning either down or up. Usually photographers will stand when they shoot the model, so the horizon line is usually at around chest or shoulder level. I did a little diagram that might make it easier :)

What a great question! I always have trouble with this. I don't like guessing so let's figure out a logical method to at least get us close. 1. What is obviously above the horizon line and below the horizon line? 1a. The feet are obviously below but I don't see anything obviously above. If the head was way above I would see the underside of the chin when the head is level. 2. Can we get a bit closer with some other kinda obvious points? 2a. The boxer shorts on the guy have a line that is curving downward in the center. That tells me that the horizon line is above that point. A bit closer.... The problem is that I can see things that indicate they are below the horizon line but I can't find anything that clearly indicates it's above the horizon line. I know it's above the ASIS... If I have to guess from here I would say that the horizon line is above the navel and right around the chest region for both models. I wonder, if perspective is this close to being neutral if it even matters. Just draw the figures as if the horizon line is going right through the center. I'm not sure if my rambling helped but there it is. Thanks for the question!

 You have to follow those perspective lines that you find. So if you don’t know perspective you will have to learn the basics to see it. To me those pictures look like the camera is about where the abs are for the guy, and where the chest is for the girl. So that is my best guess.