In my experience, hardware diagnostic utilities will identify only the most obvious problems.
If the battery is charged, try running these tests using only battery power.
One test I use for the optical drive is to insert a DVD with lots of files on it (I have test disc) and drag-copy it to the HD. If that works well, I then burn a CD using some of those files. This won't tell me if the drive will successfully burn a DL-DVD, but I don't want to do so many tests that I have to pay rent to the guy.
Give the processor and its fan a workout by secure deleting some of those copied over files.
Bring a mouse along to test the USB ports. Maybe a pair of headphones or earbuds. An external Firewire drive and cable if it has FIrewire.
If the seller believes the laptop to be free of issues, he/she shouldn't mind you performing the tests. Just explain what you would like to do before you start. Offer to buy the seller a latte if you're at a coffee house.
Good luck. Let us know how it turns out.