I try to find out what the highest point is of a 2d geometry that includes polylines and curves.
I would say that it is pretty is for polylines by a simple for loop, however I am not sure about curves.
Is there any method that tell me the highest point of a curve (or spline)? (or I need to check math equations?)

A fairly good approximation can be to generate a point VERY far away from your geometry, then pull it to the geometry. The pulled point should be near to the exact solution.