Our BEST filter on our guild app is "Post a screenshot of your UI!"
We get a ton of people applying and will find me in game asking me things like "What is a UI?" and "How do I take a screenshot?" These people are denied as we are clearly not the guild for them nor they for us 99.999999% of the time. Also, the people who get past those two questions and then submit the screenshot link as "C:\My Documents\WoW\ScreenShots\12345.jpg" or something similar.. Gives us all a little laugh in members-only-chat as we deny their application.
(Note: We are a Progression 10M Guild on a semi-small server and we enforce a 18+ age requirement and previous progression raiding experience due to the nature of our guild environment and the topics of conversation that pop up on a daily basis. It limits our pool of applicants but it keeps us (and them) safe.)