The main thing is to go and hold the unit, and then see if your thumb is riding up into the path of the string.
http://www.youtube.com/watch?v=vtoKbjbRDxwThis is why some of the newer units come with thumb/finger shields or other mechanism to mitigate the risk.
I found out about this when I was at Cabela's last year shopping for a crossbow, and heard the story about the guy buying a crossbow to kill a big buck that was showing up on his camera during archery season.
The end result was he wife made him take it back the next day after he removed his thumb
..I don't think he got the buck either
Also, there are some reports that Basspro was sued by customers getting injured while test firing a crossbow.
In the end, I did buy a Wicked ridge CLS crossbow (performance was ok), but only after taking the whole family to Cabela's and having them handle various crossbow options.