Easiest way is to shine two flashlights in the same direction so they combine. Cover one flashlight with red and one with green cellophane. The red and green will add up to yellow.
You probably can't distinguish it from yellow light in the visible spectrum by your eyes alone (unless the red and green are not very saturated colors themselves). But you can distinguish it if you have more colored cellophane! Look at the yellow light through red or green plastic and if you get red or green, then it is from the mixture. But if it still looks yellow through each of the red and green filters, then it is a pure yellow from the visible spectrum.
You could also tell many other ways, but you'd need things like prisms, lenses, colored items or specialized equipment.
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