You will only be able to guess the sex of the P metallica in the mentioned way much later on... with each molt the line on the abdomen should get lighter if it is a female. Also do take not that with some pokie species you cant use that method as they dont lose the line.
May I also add that I cant see how this person could suspect it to be female unless a molt was sexed... and if that the case he/she would have needed a microscope to see the spermathecae cause it would have been extremely tiny.
Best way like you said is to wait a couple of molts and then sex the exuvium.
Sent with Uncle Charlie's Pokie machine
"You don't have to eat many mangos to have a nice mango, just take good care of your mango and it will be nice!"