Yeah, Knoppix is probably your best bet for this - its hardware detection is second to none.
I would use badblocks (although, I believe its ability to detect errors without destroying data is somewhat limited, and it can take a while) to check the disk, and I'd check the battery status with 'apm'. That should give you the capacity % left and the estimated time remaining. Whether this is actually reliable depends on whether the battery reports its status properly.
I would check http://www.linux-laptop.net/
and look for the model you're buying to find any problems.
P.S. I'm having a lot of fun with my new (second hand) Thinkpad and Fedora C2.