Underside of a Tliltocatl albopilosus Nicuragua sling, magnified with a usb microscope. Apologies for the numbers, this little one was hanging on the side of the enclosure which is a repurposed container that for some reason has those measurements. Hope it's clear enough.
When they're that size, any external differences between males and females are going to be very challenging to spot. You'll have much better results if you snag a molt and examine the actual molt under the microscope. Even then, it can be tricky when they're very small. Not all of their reproductive organs are fully developed.