How can you check if a treasurer was properly elected?

You would find out through body corporate records, such as minutes of the annual general meeting. As an owner you are entitled to search records and obtain copies. You do this search through your body corporate manager, if you have one, or through your secretary.

That, of course, presumes that (a) meetings have been held and (b) minutes have been made and kept as they are meant to be made and kept. If either (a) or (b), or both, have not occurred in your situation, then something is amiss as it is not an option for someone to simply assume a committee position or declare themselves to be Treasurer. People are either elected to the positions, at an annual general meeting, or appointed as a casual vacancy.

Your scheme may need an administrator appointed, if no committee members have been validly appointed, or there are no records being kept.