You can tell you have a split-head mattress if it is physically split from the top to the center (see illustration for details). These mattress types are specifically designed for adjustable bases in which both sleeping partners have the ability to adjust the incline of their side of the bed to their individual preferences.
On the other hand, a split mattress is nothing more than two mattresses that either sit on two separate adjustable bases or share a single bed frame.
Example: (1) split-king mattress is equivalent to (2) extra-large twin mattresses on a single king-size bed frame. This is why if you purchase (1) set of Split-King sheets, you will receive (2) Twin XL sheet sets in your order.