One way, run "winver," a program that ships with Windows, C:\Windows\System32\winver.exe.

Windows Key-R:

Screenshot from Windows 10 Pro Version 1909 Build 18363.836:

In Windows 10, 8, and some Server versions, you can find the version and build information on the System -> About page in the Settings application:

A shortcut to that settings page on Windows 10: Right-click the Start Menu (or Windows Key-X) then choose System: