Is my 2 prong outlet grounded?
Your house is grounded but doesn’t have an equipment ground. How a 2 wire system works (120v) is one side is actually grounded and the other is at 120v. A 240v receptacle has 2 each 120v legs that are 180 out of phase but single phase or split phase is the proper name for residential us power. So your maximum voltage to ground is 120v , leg to leg is 240. The common leg is grounded at the transformer that supplies your home and is called the neutral. If your receptacles that show a voltage are switched it is normal to measure a “phantom” voltage of 40 to 120v phantom voltage is induced into a wire that is not powered by running parallel to a live wire this is how transformers work. If you plug a load like a light into the receptacle the voltage will normally drop to zero. 
Phantom voltages can be found in just about any home. 
To have an equipment ground or a 3 prong outlet that is functional you can add a ground wire to that circuit this is a recent code change. The other way that you can add 3 prong receptacles is to add GFCI protection and label them GFCI protected no equipment ground. 
So your system has a grounded conductor by code the white or gray colored one but no grounding conductor the bare or green one. Hope this helps.