Depending on our freedom from delusions, we can recognize conditions of equal or lesser purity.
It's like looking through glass: if other's glass is dirtier, you can see that, but if it's cleaner, your own dirt doesn't allow to recognize that.
Also, it's not possible to get consensus among practitioners, because many of them have delusions and misrepresent their qualities and qualities of others. Some teachers disagree with each other, some claim awakening out of ignorance, some have large following despite poor qualities etc.
In some Buddhist traditions people try to establish systems of quality control based on opinions of authorities, but it doesn't really work.
Buddha didn't set such system. He said:
Therefore, Ananda, be islands unto yourselves, refuges unto yourselves, seeking no external refuge; with the Dhamma as your island, the Dhamma as your refuge, seeking no other refuge.