With PDFViewWPF how do I detect when a user clicks on Signature field?
You could start in Pan.cs ProcessInputDown function which will get the click event.
However, at the time of this writing, Pan.cs IsWidget intentionally returns false for signature fields, so if you modify IsWidget to return true for signatures also, then you could do your clicking on signature field logic in FormFill.cs HandleWidget.
Exact location depends on what you are doing, but probably HandleWidget is the best way to go, as it would also handle keyboard input, or stylus/touch.
Pan.cs and FormFill.cs are both in the PDFViewWPFTools project that comes with the SDK.