Imagine a 2-phase sub-panel: 2 bus bars down the middle, 2 lugs at the top to connecting the incoming power from the main panel. (No main breaker here because this is a sub-panel; the disconnect breaker for this type of panel lives in the upstream panel.)
Now imagine a sub-panel that contains 2 completely independent sets of bus bars. This is a split bus panel.
Here's a picture:
Typically, in a split bus panel, the wires delivering the power to the panel are connected directly to the lugs for the upper bus. Then a double-pole breaker is installed on the upper bus and wired to the lugs on the lower bus.
This is allowed because of the "6 breaker rule", which says that a panel must de-energize all of its circuits by switching off at most 6 breakers. Provided that the upper bus has at most 6 breakers installed (say all double-throw breakers for big appliances) and one of those 6 is the double-pole powering the lower bus, then it takes at most 6 breakers to de-energize all of the attached circuits for the entire panel.
So you can identify a split bus panel by looking in the middle for a (ahem) split bus. It should be obvious because there are lugs buried between the breakers, and those lugs are likely wired to a breaker above.
(In looking for pictures or diagrams to help me explain this, I came across a website from Charles Buell, a home inspector, that explains a lot about split sub panels, including some mistakes he sees, plus some things that people claim are mistakes but are not. It's a good read and it might help you.)
