I also do the "look and see" method of prewashing. If the color catcher is really dark I will wash again, or double check the piece I'm suspicious about. You can rub damp cloth into a white paper towel for example and see if more dye is being cast off.
I've only had problems once and it ruined a quilt, the maroon dye was not set properly. I had a sophisticated maroon, silver/grey and blue color scheme with sparkles of white. I prewashed the fabrics and didn't think about it but I always wash quilts before giving them away. To my horror not only were the squares of white now baby pink but a few pieces of a poly blend white had snuck into the top and they stayed bright white.
Here's a picture of the offending fabric. They both started out the same shade of maroon. The darker one has been washed at least twice (once by me), I found it at a thrift shop. The "orange" piece was the same color and is the same fabric as ruined the quilt. The dark one I used in my original quilt in the blue colorway -- which was colorfast and had no problems. The lighter one has been washed at least 6 times now and still runs like a sprinter!