When To Know If A Guppy Is Male Or Female.

I’m sure others will be along very soon with much more experience but from what I do know the females often have super coloured bodies and are much more drab in colouration overall. I think their larger too but this is probably not helpful if juvenile.  Wait for other before you make a firm judgement! I’m no expert as I’ve never kept them personally!!

I used to have endlers, back in the days before I realised my water is too soft for them 

All livebearers look female until puberty. Then with males the first ray of the anal fin begins to elongate and the rest of the fin shrinks. A magnifying glass is useful for spotting this early. It does take at least a couple of months though. I would wait a while longer yet.
Males tend to be more colourful but you can't rely on that as there are colourful females these days. And females tend to have small round tails - but so do males until puberty.

Guppies can be easily sexed from about 4 weeks old. A male will develop its gonopodium at this age, and start to develop its colours, and a female will develop a gravid spot just behind the anal fin.
They become sexually mature at about 6 weeks.