Identifying the triangle to which the given fragment belongs

Suppose there is an instance of ofMesh whose mode is OF_PRIMITIVE_TRIANGLES. The mesh is a polygon made up with some given vertices and indices.

I’m wondering if there is any way to know to which triangle the given fragment belongs in a fragment shader?

perhaps it can be useful to look at this:
https://www.opengl.org/wiki/Tessellation_Control_Shader
https://www.opengl.org/wiki/Tessellation_Evaluation_Shader

and: