[mshac]

Test the pressurization to be certain it can maintain max differential. If there are leaks, they can be expensive to find and fix.

If there is a recording engine monitor like a JPI, download the data and send it Savvy Aviation for analysis. They can really give insight into the health of the engines and individual cylinders.

Check for oil leaks on the rear engine. They are SUPER common.

Check the plane after sitting overnight or even better a few days to see if the gear doors start to droop down. If they do, your gear accumulator is bad. Its located under the floor near the pilot's seat.

Test the gas-fired heater and be certain its been inspected on schedule. Surprising to me how often this gets overlooked, especially in warm climates.

Check to see when alternator drive couplings were last changed. Mine had just been replaced at about 800 hours when I bought my P-model. They typically wear out before TBO. If they fail, they cause serious internal engine damage.

Test flight for fuel flows (are they matched?), throttle positions (are they aligned?), oil pressure/temps, make sure all gauges have correct indications. Check the AP for holding altitude and heading. Check the NAV function on both a VOR/ILS and a GPS fix (if coupled). Check the electrical system by turning off each alternator to be sure the other alternator picks up the load. Check to see if the alternator warning lights are blinking during flight (another very common scenario that my 400-series voltage regulator upgrade is designed to fix - $499).

Check fuel gauges for correct indications throughout the range - they are a known weak point in the Skymaster.

Be sure the powerpack is topped off with hydraulic fluid (its behind the center pedestal cover).

I'm sure I missed some things, but this is a start at least.