for those comfortable with the command line, I've been installing smartmontools on every OS X machine I have. It has a daemon that will run the self tests on a nightly basis, and is a nice thing to have incase you start running into a problem.
once it's installed, you can do a smartctl -a /dev/disk0 and get the list of the self tests that have been done, and their status. For example, on my work machine:
MART Self-test log structure revision number 1
Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error
# 1 Short offline Completed without error 00% 6541 -
# 2 Short offline Completed without error 00% 6519 -
# 3 Extended offline Completed without error 00% 6498 -
# 4 Short offline Completed without error 00% 6496 -
# 5 Short offline Completed without error 00% 6474 -
# 6 Short offline Completed without error 00% 6452 -
# 7 Short offline Completed without error 00% 6429 -
# 8 Short offline Completed without error 00% 6407 -
# 9 Short offline Completed without error 00% 6385 -
#10 Short offline Completed without error 00% 6362 -
#11 Extended offline Completed without error 00% 6342 -
#12 Short offline Completed without error 00% 6340 -
#13 Short offline Completed without error 00% 6318 -
#14 Short offline Completed without error 00% 6295 -
#15 Short offline Completed without error 00% 6273 -
#16 Short offline Completed without error 00% 6251 -
#17 Short offline Completed without error 00% 6228 -
#18 Short offline Completed without error 00% 6206 -
#19 Extended offline Completed without error 00% 6185 -
#20 Short offline Completed without error 00% 6183 -
#21 Short offline Completed without error 00% 6161 -
I've currently got it running on a PM G5 at work, a iMac G3 and a mini at home -- works like a champ