If I mask an SVG, how can I know if tha mask actually has something?

I have a project in which I need to recreate an SVG and animate it given some positions (x,y), and I am supposed to know which elements of the SVG are drawn in a certain position.

My first thought was to mask the elements of the SVG in the position I'm given. But how do I know if there's actually something in the mask? Because it could be empty right?

I have to do this using C++. Can anyone help me with this or suggest another idea?