1) Aikido FAQ. This is an excellent source that new-comers to aikido need to be refered to. It is quite through and answers most questions, but takes a while to get through. I remember when I first looked into aikido 3 years ago, I spent a week just reading everything on this web-site and STILL never covered it all.
2) A "Caution" sign in dojos. Members of bone fide organizations could come up with a poster/sign that explains who/what they are, what affiliation they are with. Have a second paragraph that explains that many styles of aikido are out there and that there are also "schools" that are not legitimate... that the students should feel encouraged to ask question. AND, where the asking of questions is not welcomed, that this should be taken as a sign to keep looking.
Just my 2 cents.