There may be better sources on the web, but Roger Tory Peterson's Guide to Birds of Western North America is THE authority in my book. It clearly points out the difference in field markings that help clinch the species ID. Get a field guide book and you'll learn to look for the slight but apparent differences in size, plumage, behavior, voice/call of these birds. A high quality pair of binoculars and a spotting scope and tripod stand will serve you well for birding year-round.

It is not impossible to see multiple hummingbird species. It is however difficult to accomplish! Therein lies the challenge and the satisfaction of first-hand discovery. It is possible!