Well first off, tell me what kind of powder its supposed to be, what it looks like (fluffly, crystals, fine powder, granules), the color (pure white, off white, caramel color, redish, brownish, be specific), if it has a strong odor, and any other physical attributes you can mention. Does it melt in your hands (in the baggie).
Take like 100mg and put it on some aluminum foil in a preheated oven and check that the temp is like 10 degrees COOLER than what its supposed to melt at. Then put it in and make sure it doesnt melt, check temp again. BTW do all the checking of melting with the oven OFF because it heat cycles.
Next bump up the temp 10 degrees and turn the oven off when it gets there (check temp) and put the sample back in. Wait a few minutes and check for meltage.
Do the same thing, bump up 10 degrees, check heat and put in sample with the oven off. Check for meltage.
If its MP is 300 degrees and it didnt melt at 290 or 300 but melted when you tried 310, its close enough and by referencing the physical propeties you can be pretty sure.