Cut the fabric up into swatches, perhaps three or six inches wide, whatever's small and convenient. Using zip-lock-type plastic freezer bags (not the storage bags which are too thin for use in storing frozen foods), place one piece of fabric in each bag. I like to use quart-sized bags for this (i.e., one-liter bags or smaller). Mix a small amount of each fiber reactive dye that you want to test with a small amount of water and pour it over a swatch of fabric in the bag. Also mix up some sodium carbonate, one teaspoon (5 ml) of soda ash per one cup (250 ml) of water, and pour some of this into each bag. Seal each bag, pressing out most of the air.

Do you have a microwave oven? If so, you can microwave the bags, all at once, inside a dish of some sort, watching closely until the bags puff up with steam; stop the microwave before they can explode. The bags will slump down as the steam condenses again. Repeat this if you want to be sure the dye's good and hot, then allow to cool at room temperature. If you do not have a microwave oven, then fill a bucket or a sink with hot water, at least 60°C (that's 140°F), or hotter, heating the water on the stove if necessary, then place the sealed bags into the hot water and leave them for an hour or so. The extra heat from the waterbath or the microwave oven speeds up the dye reactions, to make the test more practical, so you don't have to wait until the next day to get your results. Drimarene K dyes like extra warmth more than Procion MX dyes do, but I have used this test for Procion dyes many times.

After the bags have cooled, rinse the swatches out with cool water (a colander is handy to prevent small slips of fabric from going down the drain), then with the hottest water you have available, to see how much dye remains in the fabric after the hot water has removed as much as possible of the unattached, unbonded dye. You can place all of the swatches in a net lingerie bag to do this in the washing machine, or, for greatest efficiency, you can even pour boiling water over the fabric.