According to David C. Zlesak at the University of Minnesota:

“Rhododendrons and azaleas are spectacular flowering shrubs that add splashes of bright color to the landscape. What are commonly known as rhododendrons and azaleas are genetically distinct, but are both within the genus Rhododendron.

They can be separated based on a number of features such as scales on the leaves and the shape and architecture of hairs.

One very distinctive trait that holds true most of the time is that azaleas have 5 stamens, while rhododendrons have 10.

Stamens are the male organs of the flower and terminate in a pollen filled sac called an anther.”