informal rush

you don't because you have to be invited to the houses. Informal Rush is mostly up to the girls so if they don't invite you then you can't obviously get in. Try formal recruitment

email them!

Most chapters don't like to publicize they are doing fall rush because of the perception nobody wanted them during formal rush. The reality is there are more reasons than that why reasons houses do it----maybe they had a ton of senior graduate and have to get numbers back up or maybe people transferred or dropped. You can probably ask Panhel/Office of Greek life and then like somebody said email whoever the recruitment contact is. I HIGHLY doubt you will be turned away from attending a party---it shows you're really interested.

There are events in the fall to gain more information about greek life. You can find out whose doing informal recruitment then and possibly even receive contact info from each chapters president to learn more about their rush events. If you really want to do informal, you will be able to.