PDE Tool Box: How to extract Face or Edge Number from Data Tip Coordinates?

I have a structure work (a crane) that I import as a stl file. In order to set the boundary conditions I should know the face, vertex or edge number. Unfortunately, plotting the structure work with the face, vertex or edge number labels turns out to be very messy and it is not possible to assign or even read (because the labels are put on the top of each other) those labels. So I came up with the idea to work with Data Tips in the plotted: I click on an edge or face and the Data Tip yields the coordinate of data point (or in the vicinity, i.e. the closest node of the mesh). With

NodeNumbers =findNodes(mesh_model,'box',x_limit,y_limit,z_limit)

I can find even the node numbers within a volume. However, this is no help because it is not possible to define a set of nodes for a boundary condition (i.e. constraint fix)! I need the face or the edge number at the locations where the boundary conditions are set. So, how can I extract the face and edge number(s) which is connected to the coordinate of the Data Tip?

Thanks and best regards

Urs