The best answer is 'don't bother to try'. Earthworm identification requires a low power microscope and mature specimens. If you haven't been put off yet then you will need a guide to help you - there are some 30 species. There are quite a few books available but only a couple worth reading. It is worth noting a little about earthworm habitats.
Earthworms have no means of regulating temperature or water loss.
So if conditions become too extreme they can not stay active. Other species die off but leave eggs in cocoons which are better suited to survive adverse conditions. Similarly in winter earthworms burrow below the frost level and remain inactive. Earthworms and lawns Some gardeners want to eradicate earthworms from their lawns, usually because of the worm casts. So the advice is, don't try killing earthworms in your lawn, it may give you a better lawn in the short term but in the long term will lead to poor soil quality.