Instructions
Check the website for the city or park that the mountain range is associated, or an organization or agency such as the U.S. National Park Service that has information on various mounts. For example, Mount Whitney, which is taller than 12,000 feet, can be researched through the National Park Service, Sequoia National Park, Kings Canyon National Park or other sources dealing with the Sierra Nevada mountain range.
Call or write the city or park associated with the mountain. The website should have a link to contact information.
Check encyclopedias. An encyclopedia will give you details about the mountain range, including the height. Peakware.com, for example, is an online encyclopedia that focuses exclusively on world mountains. The Encyclopedia Britannica, which has been published for decades, is now available online.
Try an atlas. It includes the same type of information as that on a map, including a mountain's height. World Atlas.com is an online atlas on which you can look up a mountain range.