You can take the free NBME test at Prometric as a practice session. I went through it there. I think you can d/l it, but it didn't work on my mac when I tried.
There's a part where you have to check your equipment. I feel like no matter what, people should go through that section. There was a girl during my practice session who didn't, freaked out when her headphones didn't work, and ended up getting reported for misconduct to NBME. So, you know, don't be that person.