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There is not a very easy way, but there is a way. More than one actually. Take a swatch and place it in a beaker of water overnight, but make sure the beaker is covered. When that's done put a pH test strip in the beaker and wait a few minutes. The greater the color change, the stronger the pH.

get the fabric, cut it into tiny tiny pieces, put in some pure water and perhaps put this mixture into something to mash it into even small pieces. Then test that for pH.