To check if a fabric is color fast you need to soak it. If it is a precut you can lay it on a paper towel to dry or hang it. If color drips or ends up on paper towel it isn't color fast. For a larger piece of fabric, soak it in water. Take a small amount of water in a clear container and look at it. If there is only a small amount of color, color catchers will take care of it. Occasionally a fabric bleeds heavily. Then you can soak it in Retayne for 20 min. and then rinse or wash out.
The difference between Retayne and Synthrapol is a quote from the New Pieces Quilt Store and Gallery site
Retayne FIXES dye to fabric and should be used in cases where you are concerned that a fabric dye may bleed. Use "Retayne as directed, as a pre-wash or before putting suspect fabrics into a quilt.
Synthrapol is a special detergent that suspends dye particles so that they do not reattach to fabric. Dyers use Synthrapol as a pre-wash to REMOVE sizing, oils, fingerprints and other impurities that would interfere with an even dying of fabric — and then they wash the dyed fabric with Synthrapol to remove any remaining unattached dye particles."