First you need to get the world position of the bone's tip.
A naive way to do this is to start with a vector that is length, 0 using the bone's length, rotate it by the bone's worldRotationX, then translate it by the bone's worldX, worldY. The vector is now the world position of the bone's tip, but it will be wrong if the bone is skewed by nonuniform scale of a parent bone. The right way to do it is to use bone localToWorld, passing in length, 0.
Now you have a line segment representing the bone which is from the bone's worldX, worldY to the world position of the bone's tip.
Next, you have a circle whose origin is the touch position. The radius of the circle is the touch distance.
Now all you need to do is line-circle intersection to see if the touch circle hits the bone line segment.
