Is it useful (and polite) to ask a potential meditation teacher about their attainments?
Worrying about whether something is polite or not is a hindrance. I am sure some practitioners would find it offensive but if they are offended that probably tells you all you need to know.
If you have yet to obtain the Jhanas then there are probably others who are still more advanced than you who have not yet obtained them either. The way I see it, anyone who is beyond you can be a good teacher and anyone who you are beyond can be a good student.
According to the Thana Sutta:

It's through living together that a person's virtue may be known, and then only after a long period...
It's through dealing with a person that his purity may be known, and then only after a long period...
It's through adversity that a person's endurance may be known, and then only after a long period...
It's through discussion that a person's discernment may be known, and then only after a long period...

You can't really know simply by asking.