When a node is dropped on to a document i'm trying to identify 2 things; a) the node being dropped, b) if the node being dropped overlaps another node and if so the node that was hit.
a) Whatever has been dropped will be part of the current GoView.Selection. Remember that there might be many things in the selection.
b) It's probably easiest to use the GoDocument.PickObjectsInRectangle method, using one of the "...IntersectsBounds" values of the GoPickInRectangleStyle enum. You'll need to do this for each object in the GoView.Selection, and ignore those objects it finds that are part of the selection.