I have a similar situation where I want to know exactly where on a collider i am hitting and I use the coordinates on the texture that was hit to help determine exactly where on the object it was hit.

Ray MyRay = MyCamera.ScreenPointToRay(Input.mousePosition);
RaycastHit MyHit = new RaycastHit();

if (Physics.Raycast(MyRay, out MyHit, 120f))
{
string name = MyHit.collider.gameObject.name;
Vector2 pixelUV = hit.textureCoord;
if( name=="WhatImLookingFor")
{
pixelUV.x *= WidthOfTexture;
pixelUV.y *= HeightOfTexture;
...
}
}

Does that help?

you can give a spesific name to each of them and then you can recognize them with their name. if you want to see wich is clicked you should trace a ray from mouse position to see what will be collided. also you can use the GetInstanceID() to get the unique id of the collider and compare it to a pre built list of collider ids.