I look at them all. It takes hours so I have a system.
First; Divide into job categories by job type. We hire for multiple positions. I pick one job category and get going.
Second; Now, with one job in hand, I scan resume before looking at cover letter. I have to see the facts. If the experience and skills aren't there, the cover letter probably won't help.
Third; Print the possibilities. I know not every one will be an exact match, so I probably print more than I'll need. But aside from the exact match, I'm looking for that diamond in the rough. The one that might have experience in other kinds of jobs that could transfer into what I'm looking for in the employee for this job.
Fourth; Now from the 100 resumes, I'm probably down to 40 or 50. These I read, highlight, put question marks on, and circle facts to narrow down to the best. Here I'll read the cover letters. Yes, the right cover letter could sway me. Cut the fluff, just tell me why you're the best match.
Fifth: Now I might have 25 good ones. I put them in order from great to good and start making phone calls to do a short phone interview.
That the first hurdle in hiring. Next up will be that Oh So Important; Phone interview.