Why can I see the inside but not the outside of some objects?

Question

I'm running into this weird problem on my model where some objects have only their inside mesh visible, with their outside being "invisible". You can see in the screenshots below that, from any point you see, my arm is only showing it's outline, like you can only see it's inside but not the outside. Does anyone know how to fix that? This is not happening in any of the other body elements, just on the arms and the legs.

I tried using the Solidify modifier and it worked but created issues later. Since Solidify creates a "second mesh", the problem is that when animating with bones the now created inside mesh gets really weird and goes outside of the character depending on it's position, as you can see in the 3rd scrrenshot, so I'm looking to a solution without adding this modifier. Thanks in advance!

The issue in my model's mesh:

The bug that solidify creates on my legs when animating it:

Answer 1

In Edit Mode go to Overlays (top of your viewport, towards the right hand side) and check 'Face Orientation'. The parts with inverted normals will turn red. Then you can go to Mesh, Normals and Flip or Recalculate the affected parts of the model.

Answer 2

When I was importing an .fbx everything was flipped. Turns out that the material that was applied had Show Backface turned on under the Settings drop down inside the Materials tab.