Reptile vs. Amphibian

• Check the skin. The only likely point of confusion in the western United States in this regard will be between salamanders and lizards, both typically slender and long-tailed with four stubby legs. Salamanders, which are amphibians, have moist, scaleless skin and no claws, unlike lizards.
  
  

Classifying to Order

• Most reptile and amphibian groups are easily identifiable. There are two species of legless lizards in the west, which superficially resemble snakes, but the former have movable eyelids. Frogs and toads look similar, but the latter typically have pebblier, "warty" skin and are more likely to be found well away from water.
  
  

"Keying" Species

• Whittle down your ID to species by noting detailed characteristics. These can be quite subtle, and a good field guide is essential. Snakes, lizards and frogs especially can be quite fleet, so getting a good, solid look at their features is often the major challenge. In general, coloration and pattern should be your top priorities.
  
  

Range and Habitat

• Know where you are. Using your field guide, cross-reference a likely candidate's normal geographic range with your own. More specifically, take careful record of your location: Are you in a dry wash, lined with cottonwoods? Are you in a quaking aspen stand in the highlands? Are you in a sagebrush steppe? Then check the habitat preferences of candidate species to narrow down the options for your sighting.
  
  

Warnings

• Take the same standard precautions you would around any animals with reptiles and amphibians. A few western snakes are venomous: the rattlesnakes and the western coral snake, all of which are usually readily identifiable (the former by their eponymous tails, the latter by its striking, "red-on-yellow-kills-a-fellow" coloration). The Gila monster, a large, robust lizard, has mild venom and powerful jaws, and shouldn't be molested. But then again, no reptile or amphibian should be unduly disturbed.