Not necessarily. For example on Amazon, I just checked the Notebook section and there are offers with Linux preinstalled.
But I agree, if someone can spell it (like, no user that says "I click this symbol to use the Internet!" when double clicking a webbrowser shortcut) and tell you they're on Linux, I'd say the chance of them actually being on Linux is like, 99.9%.
What distribution are you using? Does Steam and other Steam games generally work without problems?