The Onload stackdump utility exposes counters to indicate how often TX PIO is being used - see Debug and Logging. To view PIO counters run the following command:
$ onload_stackdump stats | grep pio
pio_pkts: 2485971
no_pio_err: 0
The values returned will identify the number of packets sent via PIO and number of times when PIO was not used due to an error condition.