I want to make a game using these assets. Is this legal?
Not sure as to Sketchfab but anything made available under the Epic umbrella is fair use. By extension anything covered by Creative Commons Zero or MIT is also fair use.
Any asset store should list license information either as the asset itself or somewhere on a dedicated page.
For sketchfab your intended use case should be possible 
Free assets in sketchfab are using one of various Creative Commons licenses that is displayed (and explained) in the products page.