This can be done temporarily using a temporary shaft power measurement system which installs direct to the propulsion shaft and running engine tests to full power.  The data can be logged and stored as evidence.  The Datum Electronics Shaft Power Measurement Kit is the kit of choice by many EU fishery agencies and is simple, easy to install to prove your engine power unobtrusively and without the need of a team of technicians as it has been designed to be installed direct by customer.  The SPMK system can be installed in less than 30 minutes and removed from shaft after your tests are done with a log file for you to keep.