Serums containing "ascorbyl palmitate," "tetrahexyldecyl ascorbate," "magnesium ascorbyl phosphate,"...etc. are all legally allowed to be called "Vitamin C serums," though these derivative forms have little-to-no actual Vitamin C antioxidant power on their own. The ingredient you're looking for is l-ascorbic acid, which is the pure, natural, full-potency form of Vitamin C you'd find in a freshly picked orange. This is also the form that will show the greatest visible benefits to your complexion. We include some derivative forms on top of l-ascorbic acid since your skin has some capacity to store these forms for later conversion to the real deal.
The second thing to note is that if your Vitamin C serum doesn't smell sweet and looks like light yellow apple sauce, it's no longer Vitamin C. There's a lot of talk out there about "stabilized Vitamin C" created by adding water or ferulic acid. These indeed slow degradation a bit, but if the serum is bottled with oxygen exposure, every hour it sits on the shelf it will continue to oxidize, degrade and lose its effectiveness significantly. If you read the fine print of most of the popular ascorbic-acid serums on the market, you'll see a plea saying "don't stock up."