To determine the state of an input port you will read the relevant data bit from the address specified in Table 5 for the port or set of ports you are interested in. A '0' indicates a LOW, and a '1' indicates a HIGH input. Further, you have the option of setting ports P24-P30 as a "triggered" input meaning, whenever the state of one of those pins changes you will receive an interrupt. You can read more on this in the Transition Detection section of the datasheet on the bottom right of page 10.