1. Advanced stats, specifically PER (sorry, haters, it's the best metric we have right now for measuring players statistically) in conjunction with objective observation to evaluate intangibles (defense, hustle, how a certain player impacts the game, etc).
2. Playoff consistency. Unfortunately, not every great player will always be on the best team. For example, Lebron James in '09 (who was historically great in the post-season that year) and Kevin Durant last year had two amazing playoff performances that should boost their legacy but will be largely forgotten since it didn't result in an NBA le.
3. Finals MVPs.
4. Regular season MVPs.
5. Other individual awards (should be taken with a heavy grain of salt, though. The media who votes on these selections has a habit of favoring stars and basing their vote on a player's reputation rather than on merit.
6. Rings. Team accomplishment. Only dip fossils stuck in the 70s would give rings majority weight when evaluating a player's legacy. If rings are the be all, end all, why isn't John Havlicek, a first option player, with his 8 rings the consensus GOAT? Or why was Jerry West, who only won one ring, considered a top ten player in history for the longest time? If I recall, I think he was actually once considered the greatest player of all-time.
7. Career stats.
8. les outside the NBA.