I don't pre-wash fabric. Saves me a lot of time! However, I do test suspicious fabrics for potential bleeding, as I do not want to put a bleeder fabric into a quilt. To test I just cut off a 2-inch square of fabric, drop into a glass of water and watch to see if any dye seeps into the water. I also will rub the dampened fabric against white fabric to see if any dye "crocks". In my experience, something like 99.8% of all fabrics do not bleed; however, it only takes one bleeder to ruin a quilt.