You can measure packet retransmits from the client to the server by counting the number of duplicate sequence numbers.

Packet retransmits from the server to the client can be measured by counting duplicate Ack numbers.

Note that a retransmit is triggered by more than just total loss (= timeout); if the remote machine rejects the packet, or the packet is corrupted, the local machine must also retransmit.

via networking – how to calculate packet loss from a binary TCPDUMP file – Server Fault.

I needed a way to measure this on a wifi network where packet loss can be very high and bursty.  This answer seems relatively simple to implement — just store off  ACK sequence numbers into an array of limited size and count how many times every new ACK matches in that array.  By monitoring this count I can determine where and when certain areas are getting bad and perhaps alert or alarm based upon a certain threshold.