Well the input must be a generic object (I've asked David for a generic OnObject/OnGeometry input option), and then you can test the type. There are only a few methods which almost every object implements, and one of those is GetType. What I do (which is probably slightly off) is get that type and convert it to a string, then test that against what I'm looking for. Like so...
Dim inputType as string = myInput.GetType().ToString
If inputType = "RMA.OpenNURBS.OnBrep" Then
'Its a brep...do something
ElseIf inputType = "RMA.OpenNURBS.OnCurve" Then
'Its a curve...do something else
Else
'Its not what I want...take it back
End If