As a newb to RIU I know my opinion is likely to be seen as
.. But most plants wont tell the untrained eye until at least six weeks out of the medium..
Males IMO are much easier to spot than females in the early stages. But with females there is no mistaking, so if not too sure what to look for at 3-4 weeks wait for pistils if you think its a female.
Males will look almost like a claw or a claw giving you the middle finger, whereas females will look like a little Rugby ball pushed into the node.
If you have anything capable of magnifying 30x or more you can see which you have much earlier with a bit of practice.