Good question hopetoquilt... I would say that if you washed them with a white t-shirt and the shirt is still white that is good, and you are all set.
As to others who are in question about the washing - as the swap requires that all fabrics used must be washed. That was a really good idea that hopetoquilt used - is to wash them with an old white t-shirt (wouldn't want you to ruin a good one), and if the shirt is still white you're good to go. Also, a product called Color Catchers that are found in the laundry section, is a good way to tell. Some colors, like the dark Batiks (reds,dark greens, dark blues,etc.) may need to be washed 2-3 times for all the excess color to run clear. Batiks are especially known for having extra dye in them.