Visual gender determination of Pseudotropheus demasoni is extremely difficult because the two genders can be near identical in appearance (and aggression). Full grown males tend to get larger than full grown females and the subtle fin visual cues (tapered pelvic and anal fins and somewhat proportionately larger pelvic fins) are slightly more evident.
Visually cues among males that aren't at max size (4"), or among identically-sized males and females (or larger female, smaller male), is very difficult.
For absolute gender accuracy these fish must be physically vented (and must be near mature or at full maturity to be precise).
However, all of that might be
completely irrelevant because I don't think those fish are pseudotropheus demasoni's. They share a superficial semblence to demasoni's by being blue/black vertically striped fish but that's about it.
Demasoni's have
thicker stripes and
less stripes than the fish in the photo's. Usually 5 black stripes and 5 blue stripes (10 total stripes). Some specimens have stripe imperfections so sometime you'll see +/-1 or 2 stripes from the normal 10 stripes, sometimes half or merged stripes.
But I count about
15 stripes (give or take) on both of those fish. That's
way too many stripes for a demasoni. And again, the stripes are narrower than typical demasoni stripes. I think those are most likely some sort of
pseudotropheus elongatus or a hybrid of either elongatus or demasoni lineage. I'd personally return those fish if the intent was to stock demasoni's (and more than just a pair when you do get them).
Here are some examples of typical demasoni stripe patterns for comparison:
Small juveniles
1-day old demasoni fry