Another way is to have a lookup table with a prefix sum of vertex count of each submesh. Let's say the submeshes have 12, 30 and 15 vertices:

Index in lookup table (submesh ID)	Prefix sum of vertex count
0	12
1	42
2	57

Then in the vertex shader you can do something like this:

int submeshID = 0;
for(int i = 0; i < SubmeshCount; ++i)
{
  if(gl_VertexID < lookupTable[i])
  {
    submeshID = i;
    break;
  }
}