True Center is the dead middle of the ski, so there is equal amount of ski in front of and behind the binding.
Core Center is when the midsole is right at the narrowest part of the ski.
Many skis have Core Center marked but you'll usually have to measure to find True Center, it depends on teh ski.
"pee on the zombies, because not even zombies want pee all over them" - ktwofreeskier
"Never before in my time at the bar or on the bench have I ever had to deal with somebody who voluntarily allowed himself to be buggered by a dog on the public highway. Frankly it is beyond most of our comprehension" - British Judge