Pressuize the rail with air and stick under water, looking for bubbles. A larger plastic storage bin works.
or
Just pressurize with air (30-40 psi) and use an attached gauge to see if pressure stays true for an hour or so. I've used this method but it's a little more effort since you'll have to make sure the supply line and guage connection has no leaks. With the dunk water method - just discount any bubbles visually if not from the injectors.
