Read ADXL345 documentation from previous tutorial concerning interrupts
Implementing the DATA_READY interrupt
Wire the component properly and use INT1 to trigger when a DATA_READY occurs and call a routine to read a new sample from your microcontroller.
Setup a timer allowing you to measure the time spent between to moments.
With the help of this timer, estimate the number of sample per seconds.
Check out that this number matches the one you can read in ADXL345 documentation. Change this sampling frequency and check that the measure is indeed changing.