Power down all the transistors of the chip. Make sure that all the PMOSes are tied to VDD and all the NMOSes to VSS. Also look into any leakage paths through resistors. Discharge all the caps to ground. Then measure the current through the power supplies. This will sum up to your leakage current.
One way to do this in any ckt is to keep it active(provide required inputs) for some time after certain time make the ckt inactive or sleepmode/standby mode(removing stimulus) measure the current through devices/elements for required period of time.