Just extend Erik's example to check a particular bit for change and to only read the register in question once per check:

#define BIT4 (1 << 4)

unsigned char byte;
unsigned char old_byte;

byte = reg; /* Only read register once per check */

if ((old_byte ^ byte) & BIT4)
{
    /* Bit changed. Save new state, then determine
     * exactly what the change was.
     */
    old_byte = byte;

    if (byte & BIT4)
        /* Bit changed from 0 to 1 */;
    else
        /* Bit changed from 1 to 0 */;
}
else
    /* reg's BIT4 did not change */;

Then as Dennis suggested, check your compiler's output and adjust accordingly. Keil's C51 output for the above is quite efficient.