How about setting up a membership table? At the table someone will give those that are members (and presumably you have some sort of list!) a voting card (some sort of hand- held noticable thing) that could be raised when voting.
Some PTOs get very artsy with these (cardboard hands, etc.)

Maybe members are sent a "eligibility" ticket or some other validation that would be difficult to forge/copy ahead of time that they could bring with them and must show before being given their ballot?

Our by-laws state that you have to be a member in good standing to have voting privieges. At a general membership meeting, or at a meeting held with a program (chorus, play, etc.) where there would definately be non-members, how do you you verify who is eligible to vote?