i would just rent a coolant pressure tester , from local part house , pump it up to 15 lbs and see if you can find the leak , like cam says might be head gasket , could also be intake, they have a problem with leaks
Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L)
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One of the most common problems on the GM Vortec ‘Spider’ 4.3L, 5.0L, and 5.7L engines is coolant leaking from the intake manifold... [read more] at: troubleshootmyvehicle.com