Hello,
i'm new to Inkscape and trying to check/change the rx and ry parameters of an existing rectangle. I can create a new rectangle and freely change it's rx/ry, but could not find these parameters for an existing rectangle. Where can I find those parameters? Thanks.

Pick the rectangle tool and click on an existing rectangle.
or
Select the rectangle, pick the rectangle tool...

... and you`re welcome / to the forum ,)

To be explicit:
rx and ry numeric entry should be visible/editable with the rectangle object selected, the rectangle tool active and the Controls bar visible.

Ensure you have a rectangle object selected by checking the information area of the Status bar.