As Finance Mentor said - don't undervalue the cover letter. Someone who just hands out resumes and waits for a call is probably not worth the effort.
However, during job fairs and the like, it is quite reasonable to get a resume without a cover letter - and that should not count against the candidate. For such candidates, where all you have is a resume, look for the following:
- Spelling errors - yes, you aren't looking for English majors, but if the applicant clearly did not run their resume through spell check, then that indicates a lack of attention to detail, and a lack of pride in their work;
- Side projects - working for a start-up, the candidate will be expected to wear many hats, and having a side project, however small, would have given them some experience with that type of job;
- Course diversity - same as the previous point, with the types of courses (elective) being indicative of the candidate's academic preferences;
Using those items as a scoring system, you should be able to quickly sort the resumes, and then start reading carefully from the top of the pile until you find a candidate that matches.