Use Apple's PrivateFramework "JavaLaunching" functions to determine if the Apple JRE is installed (without triggering the dialog)

AppleJRE.py

import sys
from ctypes import CDLL
JavaLaunching = CDLL('/System/Library/PrivateFrameworks/JavaLaunching.framework/JavaLaunching')

if (JavaLaunching.JLIsRuntimeInstalled() == 0):
    print "Apple JRE is not installed."
    sys.exit(1)
else:
    print "Apple JRE is installed."
    sys.exit(0)