It's like asking what's your ATM security key. No point in asking if a real ariya-puggala will never tell you.

It's like asking what's your ATM security key. You might share it with a family member because they are your family and you trust them. Just like that, an Ariya Puggala might discuss his attainment with the Buddha or with Arya Sangha. But they wouldn't discuss it with ordinary lay people. You could observe the teacher's behavior for a while and see if he becomes angry or afraid. If he does, then at most he could be a Sakadagami. But even if he doesn't show his anger, he could still be getting irritated inside. So it's difficult to tell if one has attained a certain stage of enlightenment. But it's easier to tell whether one is not an Arahath. One of the methods I use is to see if my level of understanding, appreciation and confidence of the Triple Gem improves after listening to a sermon from a teacher. Sometimes you might listen to a teacher who might have memorised a lot from Tipitaka and the Commentaries, but when interpreting the meanings, he might make you doubtful of the Triple Gem. Sometimes you see certain teachers even going to the extent of insulting the authors of commentaries and questioning the authenticity of Abhidhamma to prove their point. That's a clear indication that you should look for a new teacher.

With meditation, you can practice it and observe your mind to see if your defilements are subsiding and being eliminated. If they do, the technique works and the teacher is good.

Yes, an unenlightened teacher with a sharp knowledge can still make someone else enlightened by giving a good sermon. But it's still won't be as good as having instructions from an enlightened person. Even with an enlightened teacher, you may not be able to be successful. Even the chief disciple most venerable Sariputta thera couldn't help a certain meditation student. Only after taking him to the Buddha, was he able to attain enlightenment. Only the Buddha can give you the instructions that will suit your personality 100%.

So I'd stop focusing too much on finding the teacher's attainment and observe myself to see if the practice works.