First, "future prospect", is a claim almost any language can try to make. Unless it is a language created by a well known corporation (Carbon for example) or famous programmer (Jai or Mojo), such claims lack a foundation. A new language can really only make the argument of truly being a future prospect, if it comes from something already successful or famous.

Thus, for most newer languages, GitHub is a valid metric. Not just stars, but the number of contributors and activity associated with the repo. Other things like books on Amazon by third parties or articles about the language in well known magazines, would clearly count too. These things are measurables, beyond just hype.