• Frequently pronounces video games as "Bideo Bames"
• Will often say "I need help passing this level it's really hard" then when you look at their screen it's running Microsoft Excel 2007
• They invite you over to their place to check out their game collection and it's just every season of Frasier (still impressive)
• Their Xbox One is actually a series of tissue boxes glued together with the word "NINTEMDO" painted over it
• When you ask their favourite video game they respond by doing a series of skillful backflips, temporarily distracting you from having asked the question in the first place
• Keeps a secret diary with "I HATE VIDEO GAMES AND I NEVER PLAY THEM" written on every page

Wise words from @UmbreoClaw, do keep in mind to spot a true fake gamer is by their choice of a snack/food. If they prefer cool ranch Doritos or any category of that sort, they're a fake gamer. It also applies to choice of drinks. If they like mountain dew there a fake gamer.

Always think about reverse physiology , think what they want you to think. If they fit in every sort of stereotype there an non gamer.

 

Spoiler

especially if discord as their main contacting platform