Signs of incorrectly trimmed kites:
Poo stance (which is caused by leaning forward to reach the bar)
Kiting with arms at full reach.
Kite stalls (flies backwards)
Inability to go upwind.
General kookiness

(n.b. correct stance is straight front leg, bent back leg, straight back, centre of body weight should be behind the feet)
The first step to correctly trimming your kite
Step 1Understand that there is a maximum distance you can pull in the bar until you have reached full power. Going past this mark will give you no more power and will result in over-sheeting and cause the kite to stall (fall backwards) or otherwise misbehave. This maximum distance will change each time you fly your kite and often several times each session. Understand that the centre lines adjustment mechanism is not really about power and depower. It is about adjusting the length of your front lines to allow for correct trim and correct stance..
Step 2Launch the correct kite for the conditions. Ensure you have plenty of room and not in anyone's way. Adjust your trim strap (depower) so that the front lines are as long as the setting allows. (max power?)
Step 3 (light to moderate wind)
Park the kite at 45 degrees at the front edge of the window. Pull the bar all the way in to the chicken loop (or unhook if you have the skills to do so). If the kite stalls, shorten the front lines with the adjustment mechanism (strap, rope, cleat) until the kite can no longer stall.
Step 3 (moderate to strong wind)
Park the kite at 45 degrees and attempt to pull the bar all the way in. If you can't without being hauled off your feet, adjust the mechanism until you can.
Step 4 On the water
If you find you have to kite with straight arms, or lean forward to reach the bar while kiting with the correct stance, adjust your mechanism until you can achieve the correct stance and your are cruising with your arms bent at least 45 degrees.
You have now correctly trimmed your kite.
Adjusting the trim setting will not depower your kite. All it does is place the 'sweet spot' of the bar throw as close to your body as possible.