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The Light Test (Most Common)
Hold the stamp up to a strong light source and look through it from the back.
Soft Paper: Will have a mottled or "grainy" appearance, often compared to looking through a paper towel or newsprint. You can see the uneven distribution of paper fibers (the "pores").
Hard Paper: Will look much smoother and more uniform in its translucency. It lacks the distinct "mesh" or "pitted" look of the soft paper.

The Snap Test
Gently flick the edge of the stamp with your finger (ideally when the gum has been removed).
Soft Paper: Produces a dull "thud" or "flop" sound. The fibers are loosely packed, making the paper more absorbent and less resonant.
Hard Paper: Produces a sharp, high-pitched "ping". This is due to the specific sizing compounds used to make the paper less absorbent and more rigid.