- The size of your foot AND toes while relaxed and standing and even running, vs the size of the shoe that you are trying to make it fit into
- The toe spring (try to avoid shoe molds where your toes aim upward)
- The heel height vs the toe box level (try to keep at a "zero-drop")
- The landing or strike created when you walk or run in the shoe
- The amount of room in the toe box (make sure it's roomy - the Altra brand is one of the best in this category that I have come across)
Here's one way to test that I read about online that seems to make a ton of sense. If you are in the market for shoes, just take the insole out of the shoe, place the insole on the ground, and then step on top of it. If the toe box is too narrow, you will know immediately because your toes will be falling over the sides of the insole. When you have enough room in the toe box, the shoe's insole will be wider than your forefoot when your toes are splayed out when standing naturally.