SO you have a list of resumes for potential software development interns, what do you ask them to make sure you get the best one(s)?
In addition to the answers about "... get them to do some real coding ... " which I believe is a great thing to do I think that you need to really assess their personal attributes especially for intern positions since their time at the company is generally short and you want to make sure you get value from them (and them from you).
The best interns/co-ops I've hired in the past were the ones that were curious, wanted to learn new things and weren't afraid to ask lots of questions (and actually pay attention to the answers).
So find out what they know about your position, did they take the time to look at your website and try to figure out what it is you do. Do they have lots of questions for you? (were they curious about your company or is this just a paycheck?)
Find out what blogs/sites they read and if they have contributed to open source projects. (do they want to learn more about sw dev?)
Asking them about a tough bug they fixed or design problem they solved and see if their eyes light up because it was tough but they figured it out. (do they love sw dev or is it just a job or school assignment)
If it's a reputable school and they have decent grades then they are going to have the basic skills but the good ones love programming and want to learn more and recognize that the internship is a way to see how its done in "real life" so look for their enthusiasm to come across in the interview. See if they are jazzed to be given the opportunity to help with a real project.
I heard this from someone and liked it: Get your dev candidates to come in and spend one day with you building something - ideally of the type that you would like them to do during their work with you and you would likely want to spend the time in choosing a good task and environment to test their skills on an even playing field.
I personally would test them on fixing a bug on a large project - maybe something open source and something that you already know how to fix.
You might want to do a variant of the above.
I have hired many interns over the years. I learned a few things. For instance, resume is not a predictor of success.
I believe in doing. Put them in real-life situations. If I give you a keyboard and a Linux shell, I can tell pretty quickly whether you are a hacker or not.
What you don't want to rely on is the "behavioral interview" with questions such as "tell me a situation where you showed leadership".
Believe it or not, I actually wrote a guest article on the topic more than 10 years ago and it hasn't aged: http://www.asktheheadhunter.com/gv981229.htm
I like the "get them to spend a day at your office and do some real work advice", but that may not always be practical. I think you can get some of the results by creating a small real-code task that you can have all candidates complete before coming to the interview. You can always find out if they did it (instead of somebody else) with a few questions on the interview.
Other than that, go with your gut feeling. Do you like the person you talk to? Do you like his/her attitude? Would you love to work with him/her? To me that is a lot more important than being skilled in the technology in fashion today.