This dot you see here to my understanding is a telltale sign of male in Poecilotheria genus. The best way to learn is to research and practice here on the forums from other people's pictures. Just google "[species name] ventral sexing" or sometimes if you add "male vs female" you'll get a side by side comparison. Generally the presence of a slit between that set of lungs (white showing through) is an obvious female, and a dot or patch of hair is a telltale male. But it gets much more difficult than that for some species, and depending on size in general. Research though, google is your best friend. Sexing via molts is the easiest and most accurate way to know for sure.
@spodermin Oh okay thank you for all of that, I'll give it a go in future. When looking just now I don't see any dot and it may just be something else but idk. ATM he's only like 3 cm so maybe I should wait until he isia little older. Thanks for all the replies man.