Vent sexing does not work on adult chickens anyway.
But if you look at the vent-- just literally spread the feathers and look-- you can generally tell a layer from a non-layer. To see the difference, look at some hens that you know are laying (vent should be relatively large, and look moist and stretchy), then look at some birds you know are not laying (could be roosters, broodies, pullets that have not started laying). The vents of the non-layers should be smaller, often somewhat puckered looking, and it generally looks dry and non-stretchy.
Of course the difference in the vents has a practical purpose-- which one can stretch enough to let an egg come through! But if you check the vent of the bird in question, and it looks like a layer's vent, that is good evidence that the bird really is laying. If you check the bird's vent and it looks like a non-layer, that increases the odds that the eggs are coming from some other source.
I think that is why someone was asking for a photo of the vent, so they can see what size/shape/texture it has. It is not really a gender test, more of a laying vs. non-laying test.
As regards other methods of sexing, you could also try a DNA sex test:
https://iqbirdtesting.com/
You send them a smaple of blood or plucked feathers from the bird, they check what chromosomes are in the DNA, and you get the answer. For all birds, males have sex chromosomes ZZ and females have sex chromosomes ZW.