A vast majority of systems can be tested on the ground using test boxes that simulate radio NAV signals, handheld VHF radios, and airspeed/altitude. Our test procedures are almost always included in the install documentation, so as long as we follow that we are allowed to let the plane fly as is.

That being said, autopilot and a number of other systems do need flight tests, and so at the end of most major installs we have a test procedure that we fly with the pilot who either owns or operates the aircraft.

I am not a licensed pilot, so I never get behind the controls. My boss is though, so occasionally he will taxi and perform engine run tests with us but its the owner who always does the final flight test. This also gives them an opportunity to ask any questions they might have for me, and get situated with the new system while the installer is there with them.