Fraternity rush doesn't really begin when everyone comes back to school. It starts in the summer, when fraternities can host potential pledges under more informal rules, get to know them better and start building relationships. Many of the best fraternities do a lot of their recruiting in advance, just like the school's sports teams. So get in contact with the people you know in the various fraternities and ask them about their summer socials. This will be the first chance you have to get acquainted. It also will be at a pace that's easier to digest. Rush week on campus is incredibly intense and pressure packed. You might be able to find the place for you before the "rush" really starts. But don't make that decision lightly. You're probably going to be better off playing the field.
Ask every question you can think of, to everybody in each house. And compare answers. If you want to know how pledges are treated, listen carefully to the ways in which various people describe the hierarchy of the house. Pay close attention to any off-handed demeaning remarks. Those might just turn into a nightmare.
Fraternities are social organizations. That's the foundation of their business. But they also are an important support group for academics, extra-curricular activities, recreation, entertainment and career networking. Concentrate questions in those fields. Find out about their study table program, how many hours a night pledges are required to study, how good the house GPA is, how much pride the group takes in its grades, how many people in the house are interested in the same subject as you (think future study partners), etc. What kind of balance does the fraternity have? It might win all of the intramural contests but not have any member elected to any post in student government. Or it might have a social calendar filled with the best sororities, yet most of the recent alumni now are working at ski lodges. Determine your priorities, present and future, and look for a match.
During rush, make your best impression, like a job interview. The fraternities are trying to impress you, so you'll do best by presenting yourself in the most positive light. Dress appropriately, just a bit better than anyone you are meeting. Answer questions honestly and be frank, just as you want them to be with you, since this is a mutual partnership. But do a lot of careful listening, too. How do they treat each other? Do they badmouth other fraternities, or backstab other people in their house. If they do, you're probably going to be next.
Be as outgoing and friendly as possible, meeting as many people as you can in each fraternity. A lot of houses have a contingent of gregarious guys that they spread around during rush, keeping some of the less savory at a distance. Try to find the fraternity members that no one seems eager to introduce you to. You're going to be living with that person, just like the other guys. So don't find out about the half dozen dirtballs after you've already moved into the place.
Be analytical and logical. Don't get wrapped up in the emotions of the decision, or sucked in by the one cool guy that you think might be a great friend for you. If he really could be a best buddy, it's not going to matter if you pledge his house or not. You need to take all matters into consideration, and then make your decision based on reason and rationale. Anything less is setting you up for misery.