Test them! Really though, the manufacturer’s instructions may advise you on the weight capacity. Most poles should be able to support a porch or tarp, it’s really about creating enough tension to support the structure, but not so much that the poles will wobble or bend. You also want to avoid sagging in the middle, as collected water could increase the weight and pressure. As a guide poles hold up tent plastic, fabric-like material, which are relatively light, so they’re not for holding up wood or heavier material. Make sure they’re carefully fixed together, secured to the tent, and on a solid, even surface and they should be able to support most tents and tarps.