I have a standard test for flame retardants, which is the test in the relevant standard.Apply a lighter to the cloth/scenery for 15 seconds, if it catches fire it fails, if it smoulders and smokes a bit but does not catch alight it passes.While a flame retardation label is mandatory there are domestic levels which are lower than
theatre ones.We once had a Council who brought in some cloth to make some
curtains and were certified fire retarded but the cloth burnt quite well, so a flame test is the only real way to test stuff.