Possible HD Problem?

My Mac Mini frequently slows down for what seems to be no reason. I have the iStat monitoring software installed so I can see my CPU usage in real time, but even when it slows down, the CPU usage doesn't spike. My memory usage doesn't budge either, nor is it anywhere near maximum. The problem is best seen while using Finder, as loading a new directory can take up to 3-4 seconds. This problem has only been occurring within the last few weeks despite the fact that I upgraded the HD on my Mini over 6 months ago.

Now I'm sure those who have replaced their Mini's HD know the difficulty of replacing the drive, so I don't want to have to replace it unless I have to. If anyone knows of a surefire test to confirm that the HD is having slow reads and writes, it would be much appreciated.