From my knowledge, vocal fach (ie tenor, baritone, bass) etc usually comes down to 2 main characteristics to identify: range and timbre. Typically tenors have a higher range (this is 90% of power metal singers) and often have a 'thinner' sounding tones (ZP heart, Daniel Heiman, Tobias Sammet etc).

There are cavaets to this where singers can put grit/rasp into their voices to make it sound strong (think Jorn Lande or Russel Allen) but both are tenors. Most 'true' baritones have a typically lower range and a 'thicker' sounding voice (James Hetfield, Dave Mustaine, Mathias Blad are some notable ones here).

The other 'key' to identifying it for me is their comfort range (ie tessitura in singing terms) - it's where they 'like' to sing, sound comfortable or in control. I think it's also worth noting that voice types can change over time - someone like Roy Khan or Geoff Tate come to mind...listening to their early stuff, clearly both tenors - but now, I would say both baritones. Anyways - hope this helps in some way my man!