It is not uncommon for beginners to ask how do you tell if a Buddhist teacher is enlightened? Given the fact that some teachers have, allegedly, groped their female students or used their authority to have sex with female students, it might be a good question. Unfortunately, this is tantamount to asking how do you tell if a Christian minister or priest is Christ like?
I faced the same problem when I was a young dude. Facing my teacher, I thought, or should I say, believed, that he was an enlightened Zen master and not just an abbot (he was only an abbot). It turns out that he was a homosexual and I was not which created some tension. The other monk who was there before I arrived, was the apple of the teacher’s eye. My teacher, in other words was a predator of sorts. But none of this quenched the fire of my enthusiasm for Zen—not in the slightest. It was just another pothole in the road to nirvana.
From the side of not being a Buddha, which I was, there is no way that I could tell who had or had not achieved enlightenment.
Granted that what Gautama realized whereby he became "Buddha" is within each of us or as our Christian friends might say: the kingdom of God is within you. But do we, personally, see the state of being a Buddha, i.e., buddhatā, or see the kingdom of God within us right now? The answer is no. And why is it no? Because our thoughts, opinions and convictions act as a barrier which block our seeing.
It is with the recognition of this that Zen becomes both more interesting and profound. Somehow we have to exhaust our thoughts, opinions and convictions. They are only on the surface like small ripples on a pond. They are also leading us astray because it is all we know.
But there is still away. It requires a retreat from everyone except yourself which means that you have to rely on your self to awaken—not some Zen teacher. Now it is up to you to break through the barrier of your thoughts, opinions and convictions to see the true Mind which is unthinkable, beyond opinions and convictions. In my example, it was a mistake on my part to assume my teacher was enlightened when he was only a few yards in front of me on the spiritual path. Do any of us really need a personal teacher, someone to hold our hand? No we don't.