Today my computer crashed (Mandrake Linux 9.2) and when I rebooted, no matter how many times reieserfsck ran, and how many times I rebuilt the trees I couldn't get my distro to reboot without finding more errors on it. Finally, after I tried deleting a single partition using fdisk (because it was ext2 from a botched attempt at installing Linux From Scratch) my computer rebooted to a blank hard disk (Or at least it appeared blank when I looked at it from Knoppix).

I've got a good chunk of my data backed up so although this is traumatic it hasn't killed me. I've reloaded 9.2 on another hard drive. My question is how do I know if I can trust the old hard drive? I'd like to put it back in my machine, because this hard drive belongs to another computer, but I am afraid that it will go down again and take important data with it.

Is there an easy way to check whether or not the hard drive is bad?

If you hear any clicking or threashing nioses from the drive, it's probably kaput.

After recovering what you can from the good partitions, you could try zeroing out the drive and see if you get any errors.

dd if=/dev/zero of=/dev/hd<X> bs=512

If no errors result, repartition, format and try some heavy file transfers.

From a hard drive faq I found:

A friend of mine is using this software suite to do the 'thinking' for him.
http://csl.cse.ucsc.edu/smart.shtml
It is interesting so say the least.

Oops that one link was old, try this one:
http://www.linux-ide.org/smart.html

which leads you to this one:
http://sourceforge.net/projects/smartsuite/
sorry about the long way around, I got it from a friend.

often the drive manufacturer will have a utility to test the health of the drive and let you know what to do next (sometimes it can repair it, but I'd not trust it that far)

And how old is the hard drive? Less than 3 years ago some of them had 3 year warranty - maybe some even 5, but I don't recall.
If you use the manufacturers drive utility test, some with even give you a RMA number to return the drive and have it replaced - for FREE!
I got an IBM hard drive replaced just this way.

you can't know when an HD will fail.
I got a pretty new Maxtor 60gig that just stop working a morning after a reboot. I never got any warning so I lost everything...
This damn stupid thing is now in few pieces on the desk near me. These nice copper disk are cute :P

Sometimes S.M.A.R.T. will notify you of impending disk failure, as well as other programs. I don't know the time frame for
"pretty new," but Maxtor's hard drives carry anywhere from a 1 year to 5 year warranty, depending upon the drive.
You can check the warranty status of your drive online at the Maxtor site.

If you can get it back together, you may can even recover your data by mounting it with another OS. I've recovered data
from a failed drive with a Knoppix Live-CD. Of course, you may have voided your warranty by breaking the seal when
you opened the case. Could have damaged it beyond repair, too, from the sound of your post :}