Even with Knob and Plug system, which is a 2-wire system things can be safe as long as care is taken to wire plugs correctly. There is though a much greater chance of error. If you ever suspect there might be a problem and have a volt/ohm meter then just put the meter on Volt AC and check to see if there is voltage between the metal cases of one appliance to another. Refrigerator to Stove or Sink to Dishwasher cabinet. Homes have lasted for a long time with old wiring systems just as long as the insulators are good and you don't have things like rodents climbing through walls shorting them out. Most wiring has insulation on it and as long as the insulation is good then your protection is there as long as the plugs are wired correctly and the breaker/fuse box is wired correctly.
Most of the newer appliances are double insulated as well as tools just for protection to the users. The danger comes from older appliances and tools that have their metal case tied to one side of a two prong plug because those plugs are often plugged in wrong. If you look at a plug/outlet usually you will see most are polarized in that one side of the plugs holes or slots is smaller (shorter) than the other side. Even two conductor plugs often are polarized in that one copper conductor (plug side) is shorter/smaller than the other which is longer/wider.
The wider or longer side is supposed to be the neutral side where the shorter/narrow side is supposedly tied to the HOT/DEATH side of electric supply. The main thing I would say, regarding your home, is if you make any changes or have any changes made make sure you hire a licensed and knowledgeable electrician and not just a handyman who claims to know what he's doing. Also never trust anything. The first indication that there is a problem is if you reach for something or touch, brush up against, something and feel a fuzzy feeling like ants crawling on your skin then suspect that there may be lethal voltage present or something wired wrong and is reason to investigate further. Never touch or grab something you are suspicious about but always bump it with the outside of (backside) of your hand. Electricity makes muscles contract so if you were to touch something that was charged with your fingers or the palm of your hand the electricity would make your muscles contract and pull you into the charge.
The safest thing is if in doubt have an expert check it out or verify with an electrical volt meter if a danger exist or not.