Can a arahant recognize other arahants?
Can a arahant recognize stream winners, once-returners and never-returners?
Can a arahant recognize the spiritual attainment (and conversely, the defilement) of others?
If a arahant is able to recognize other arahants, then we may be able to reliably establish a recognized group of arahants in our time, who can be the lead teachers of the sangha worldwide.
No. It is a quality in Buddha among humans only. However, Tissa Brahma God has this quality too. Ven Moggallana was curious about this and went to Tissa god and asked him if all beings in Brahma world had this quality. Tissa Brahman said not all beings in the brahman world had it, but only few. Tissa Sutta
Buddha said there is no measuring tool to measure others' attainment level (such as one Arahat in Buddha's time that used profanities- one may still use profanity after liberation because force of habit not mind with defilement, etc )
Another sutta I remember which do not remember the reference talked about one Arahat who had never met Ven Sariputta before. They met for the first time and has conversation in form of question answers (Ven Sarriputta asked another answered). After a few minutes, they both realized that each other reached some mile stone in their progress but they didnt know which level. They knew each other by talking. From that Sutta, Arahat may have a clue about the other but not 100% sure.
Dharma spoken by Buddha should lead us and Sangha world wide.
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.
http://www.accesstoinsight.org/tipitaka/dn/dn.16.1-6.vaji.html
In Udana 7.2, Ven. Sariputta (the Arahant) tried to teach Ven. Bhaddiya without realizing that he too was an Arahant.
The Buddha saw this and exclaimed the verse, describing Ven. Bhaddiya:
He has cut the cycle, has gone away to freedom from longing. The dried-up stream no longer flows. The cycle, cut, no longer turns. This, just this, is the end of stress.
This shows that the Buddha could recognize who is an Arahant, but not another Arahant.