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.