If indeed you are looking for the “external” relative to the macro shape of the edge loop, there are a couple of ways you can do it.
1). If you mesh is guaranteed only triangles, then the 1 edge neighbor of the edge loop will be another edge loop. You can find the 2 edge loops, and then compare circumfrences. If there are quads involved, and some verts have >4 edges, then you might end up with some diamond situations what don’t make a nice edge loop.
2). If the loop is always fairly close to planar, you can easily define a direction around the loop, making it easy to decide which way is “outward” and “inward” even if the loop is not convex like a nice circle and is L shapped or T shaped.
- If your surface shape is fairly regular without too much noise, you can use the surface normal, crossed with an edge vector to test “inside the loo” vs “outside” the loop.
P.S. What is that a model of? Is it a scan or an isosurface extracted from volumetric data?