It is usually easy to tell the difference between the male mountain bluebirds and males of the other two types of North American bluebirds. Adult male mountain bluebirds have a blue chest with little to no rufous color. The male eastern and western bluebird have a mostly rufous colored chest.
Female mountain bluebirds come in two different color phases. There is the gray phase, which are mostly gray all over except for the blue on the wings like the one pictured above, or the rufous phase which have a rufous colored chest. The rufous phase female can be difficult because it can look at first glance like a light colored eastern or western female.