Depends on where you are testing the pressure. If you are testing at the fuel rail, pinch the feed and return line then shut off the pump. If it still drops right off then it its an injector. If not, just pinch the return line and try it, if it drops off then its the check valve in the pump..
BTW The pressure will drop if the return line is not pinched and this is normal.
What made you start looking at this anyhow?