bump

Eisenia hortensis (European nightcrawler) appears violet, purple or olive brown, sometimes with pale striping in segment furrows. Its clitellum is on segments 26-32, its first dorsal pores between segments 4/5, tuberculata pubertatis on segments 30 and 31, its prostomium is epilobic and its setae are widely paired.

Eisenia Fetida (red worm), is...different. ;)

To seriously answer your question it's hard to tell if a single worm is either species. The biggest difference, of course, is size and the EH is normally a darker color.
But, to tell a juvenile EH from a juvenile EF, you would probably have to be someone specially trained to tell the difference and would need a microscope.
All that said, here's a photo showing the difference between mature worms.

http://www.redwormcomposting.com/worms/european-nightcrawlers-eisenia-hortensis/