0 1 and 2 vertexs are cavity as you see
there will be a loop for every 3 vertices that for angle and if some bigger than 180 this means it is concave
how can i measure angles in this loop?
A tri will never have an inner angle>180° (in 2D or 3D). (And all outer Angles will always be >= 180°)
But maybe "Geometry" will help you further:
It has tools functions which might assist you. Whatever you're actually trying to do.
If you need more help then I suppose that you get a more specific. You don't even tell us if you're talking about 2d or 3d.