Start File Explorer, find the USB stick and make note of the drive letter. Press the Windows and "R" keys simultaneously, type "cmd" into the "Open" box and click "OK". When the command prompt opens, type or paste
dism /Get-WimInfo /WimFile:E:\sources\install.esd | find /i "version"
into the command prompt and press "Enter". I am using "E:\" as the drive letter, but if yours is different, substitute your drive letter. Once you have the version number, go to
https://en.wikipedia.org/wiki/Windows_10_version_history and that should tell you which version is on the USB stick.
Ben