one of my swordtails is swimming at the top of the tank?

I have tried keeping swords and have seen this behaviour a lot.

Just before they die.

It's also possible she's about to birth, but every time I've seen this, they usually die in about 48 hours for no apparent reason

If you have a breeding net, put her in, but if not you may be witnessing the end.

Actually in my experience with mollies and swordtail males, they are aggressive fish. There is always one self-appointed boss of the group that is constantly picking on the rest. That may be what's happening with your female swordtail, she's being picked on by somebody and she just wants to hide.
I love swordtails and have tried keeping them a few different times but have always lost them for unknown reasons. I've kept mollies for many years and they seem to be much hardier fish.

I would suggest isolating the female into a breeding container or net to keep an eye on her. Look closely to make sure that the scales are not sticking out from the body. If they are she's got Dropsy which is incurable. But if she is pregnant, you'll probably have babies in a few days! Good luck!

have you tried treating the water for swimmbladder malfuntion, this makes then appear almost stuck in the water, unable to balance, swimming on their sides or up side dowm, try getting some swimmbladder treatment from your pet shop and try that, try feeding the fish bloodworms too, these are very high in protien and helps keep the fishes immune system strong. good luck

aren't those fishes are supposed to swim at the top,check out the body shape,they have flat heads and upward mouths,dropsy will cause it to lose control,it will roll around if it does have dropsy.