How to verify that a point or a set of uv parameters are on the actual surface?

Hi
Please help.
I have imported a STEP with a cylinder. One of the surfaces (actually it is a circle) has ortogonal UV parameters (it is aligned with the x and y axis).
I need to find intersection with a line (curve). I use GeomAPI_IntCS, which works fine until intersection points are outside the circle, but inside the U min,max range and Vmin,max range. The GeomAPI_IntCS finds that these points are on the surface.
How to verify that a point or a set of uv parameters are on the actual surface and not just within the limits of the uv paramenters?

The GeomAPI_ProjectPointOnSurf and Extrema_ExtPS find that the outside circle points are on the surface.
/lars