This is not exposed through the softdevice API. The protocol will not give you any retransmit counts or similar. If an retransmit scenario occurs, it will simply keep trying until supervision timeout has occured (and link is disconnected).
What you want to do is to verify the receiver sensitivity. This can be done by using the ble_app_dtm in combination with a bluetooth tester, like the Anritsu MT8852B.
This is usually done in combination with production testing, which we have an app note on:
www.nordicsemi.com/.../77833543
