Here's what I do, after all the appropriate ground checks for control movement, (in the proper direction!) battery charged, CG correct, engine broken in, fuel system working right, etc..

Ground handling is important, so I do some taxi tests, and adjust steering and throttle, if needed. Bad ground handling can end your first test flight (or landing) before you even get started. Then...

I use a long takeoff roll, and gentle elevator pressure for takeoff. (no flaps or retract use)

Next, gentle climb to higher than normal altitude, where I do a bunch of standard "full-scale" maneuvers, such as gentle turns, and aileron rolls. Next, I slow it down and check stall behavior without flaps. When I'm comfortable with the slow speed handling, I land and take a few deep breaths. Then I check everything on the plane to make sure nothing is vibrating loose, and that there are no other visible problems.

Next flight is a little more agressive. I take off and climb high again, (without flaps) and when at altitude I cycle the retracts if the plane has them and repeat the "gentle" maneuvers above. I also cycle the flaps, and observe the handling difference. Finally, I retract the flaps and gear, and start to really "wring it out" to check for structural problems... Loops, rolls, stall turns, and full-elevator pull-ups, checking for snap behavior.

Deploy flaps and gear, land, and congratulate myself. (or curse myself, depending on what happens on this last flight.)

After a few more agressive flights, with no problems, I think it's safe to assume that the plane works. If you have the money, and safety isn't an issue, I suppose the only real way to know everything about the design is to test it to destruction. I find that the universe has a way of doing that for you, at some random time in the future.