It's pretty easy to tell the difference between daylilies (hemerocallis) and true lillies (lilium). Daylilies have a dense root system, long, strappy foliage and clusters of flowers on wiry stems. True lilies grow from bulbs. They have a prominent, upright stalk that displays both foliage and flowers.
What gets tricky is trying to tell one true lily from another -- even though they have different growth habits, look quite different from each other and flower at different times during the summer. Being able to tell them apart is important if you want to have the full lily experience -- which means having lilies in bloom from June through August! Here's a quick visual guide to the wonderful world of lilies: