Eric, the way I found out was to have a card corrupt on me. It was a potentially important photo set. I was out in slight cold and found I had two low batteries for my 20D. I went ahead and shot about 150 images and both cards were corrupted when I tried to download them. I then went to the Photorescue site and tried out the demo. If it works for your problem, and it did for mine, all you have to do is buy a license key and you have all of your images back as well as a fully functioning recovery tool that does not expire. They even gave me a free upgrade from V2 to V3 many months or perhaps more than a year later, just emailed me the info.
So just demo when you encounter your problem. I figured that even if I did encounter a problem which required the Pro version later on, it just isn't that much money, however every single corrupted card since has be fully recovered with the Wizard version and I have never had need to look further. 29 bucks is nothing when this happens. I had several hundred images that needed to be recovered from session on the opposite side of the continent. That Photorescue Wizard collected every single one of them and put them in a folder for me while I did a few chores. I want to say that this has happened to me about half a dozen times but now I do not sweat it, just get the Wizard on the job.