Good question – it’s a shame Rachael has gone as she works with worms!
Worms are different at either end – you can tell because they have a band around them close to the ‘front end’. But you’re right, it’s hard to tell.
Apparently the best way to tell is tickle the middle, and see which end laughs 😉
Hope this (the first bit, not the silly joke) helps!
Great answer from Suzi! Really made me laugh 😀
There is a great picture of an earthworm outline on the ‘Nematode-Plant Expert Information System’ at UC Davis, showing the clittelum (the band close to the front end of the worm) that Suzi describes.