The shop owner who maintains my plane personally performs a post-MX test flight in every airplane prior to return to the owner. Since he doesn't do all the work himself, I asked how he prepares for these flights.
His summary answer was:
1. Review the work order to determine everything we touched during the inspection/repair.
2. Look carefully and touch each of the affected items. He finds more problems by touching than by looking.
Stuff that is left loose (fuel/oil lines and fittings, spark plugs and connectors, heat/air ducting, sheet metal (cowlings, inspection panels) drive him nuts, in spite of all of the QC processes in place.
3. Very careful cockpit check during pre-flight, with special attention to circuit breakers, switches, indicators and other devices. For example, he checks to be sure the red/green lights are visible on the gear control panel. If not, it's probably because the day/night switch was left in an improper position or the dimmer was left in the wrong position.
4. During the test flight, he starts at the left side and works his way across the switch panel and instrument panel, being sure that everything on the airplane works (APFD, nav, com, TXP, EGT/CHT, etc.) whether or not they touched it during the inspection. He makes careful notes of all engine indications and FF, especially on the big-bore turbo'd engines, so the information will be available for tweaking of fuel controls if necessary.
He carries a copy of the pre-inspection functional test, so he will know if anything that worked when the plane arrived for MX doesn't work during the post-MX flight, and whether or not they should have fixed it.
He also attempts to return all of the pilots "stuff" (headsets, knee-boards, checklists, etc.) to the place the pilot normally uses it, rather than leaving it in a box in the bag compartment like most shops seem to do. Everything else is left in a box in the copilot seat so the owner can put it where he wants it, or (in many cases) throw it away.
I have occasionally ridden along or flown the airplanes when he was not current, and found that both of us stay busy checking and making notes during the ~1-hour flights. YMMV
and buy this.