by flexing them and looking at the flex rating on their spec/tech sheet it will also relate to the beginner, intermediate, advanced and expert level. In general, softer for ground tricks, stiffer for expert level. Since there are no valid anchor point standard or reference...flex rating is generally relative rank order within but not across various manufactures/brands.
And there is also a torsional stiffness which relates to how easily or not a board twists.
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