I have owned several Cessna aircraft that had the Robertson STOL system installed including the C182, 206, 337, 421B and 421C I have also owned them as standard aircraft.
I was approved by the CAA to carryout flight tests on all these aircraft. Mainly at max gross and aft C of G.
They are interesting aircraft to fly as the modifications do enable you to take off, approach and land at significantly lower speeds but some can lull you into a false sense of security in that some have a lower angle of attack than the norm but at a much lower speeds, so be wary in making a steep turn or orbit on final.
The leading edges on some are recontoured by means of a cuff. Wing fences rather than VGs are installed. The flaps are interconnected to the ailerons as they droop when 10deg of flaps is selected therefore the flap and aileron rigging and tension are critical and you need to refer to the rigging procedure shown in the Service Manual supplement that should be with the aircraft.
A ground check will show if the flaps extend to the same degree and at the same time. When extended check the flap guides and rollers for any play, as G says use the app on your phone. Try this at every setting as the ailerons return to near normal as the flaps extend further.
As your C of A should still be valid any flight tests should be carried out at altitude.
Fly with someone you trust and who knows the aircraft and write a proper flight test schedule covering all aspects pre-during and after flight. Choose a midpoint C of G balanced fuel load. You can take off with zero flap, you can also land with zero flap as well but they are different. Identify the flap motor CB. Climb to safe altitude and be sure that you keep the ball in the centre and reduce the speed to 20kt less than the max flap speed and SLOWLY extend the flaps and see what happens don't just select the take off setting in one go.
Don't test aircraft near the ground and don't do anything that you are not entirely happy in doing and do involve your engineer in what you are doing. Enjoy.