There's consistency of critical acclaim and consistency in a signature sound across all releases. I assume it's the former you want in which case the measuring has already been done for you by hundreds of media outlets; be it individual magazines giving top marks to every album released by an artist or a high average across multiple publications.

If however you want consistency in sound, then you either listen to that for yourself or go by recommendations and work your way from there. Slayer are very consistent. Each album is a bit different from the previous, but you still know for the most part what you're getting with a Slayer album. The opposite of this would be Metallica and I can't be bothered to type up why in full, but most of their fans will know already.