The tester indicates open ground at two of my outlets. When I test with a light probe it indicates good across the 2 receptical slots but not good (no light) between either slot and the ground. Is this enough to determine the problem?

If this is a 120 volt circuit, a correctly wired outlet will not show any light when connected between the neutral slot and the ground. Neutral is supposed to be at ground potential.

However if your tester shows no ground on an outlet, that IS a problem. If these 2 outlets are on the same circuit and next to each other, I would suspect a disconnected ground in the first outlet box or in the outlet (or junction box) before it in the chain (from the breaker). If they are on different circuits or not next to each other on a particular circuit, then you are going to have to do the detective work twice. The only easy way to do it is open each box up and do a visual check. You may have to do continuity checks if a visual doesn't show anything.

Just curious, but when you look at the receptacles are they 3 wire (two slots and one horseshoe hole per receptacle)?

If there are just the two slots, they you have an ungrounded receptacle or someone may have replaced an ungrounded receptacle with a grounded receptacle without an equipment grounding conductor.

What do you have in the device box?

I had this problem with the "grounded" outlets in my house when I first bought it. It seemed as though the previous owner replaced the old ungrounded outlets at random with grounded outlets but did not actually ground the outlets. The ground wire was attached to the outlet box so that if you properly used an adapter when needed with the plate screw into the adapter it would work, but the next guy just put in a three prong outlet without attaching the ground. I also had a problem attaching the ground wire. The ground from the cable had been cut shorter than the hot or neutral because it was attached to the back of the box not the receptacle. The ground was also brittle so it may be a ggod idea to plan on pigtailing a new piece of ground wire to the existing to keep swearing to a minimum. Good Luck.