But you and your loved ones may be living in ignorance of the risk of bad electrical work.
Doesn't that seem a little illogical? At least you can look for and test many unsafe conditions.
Three are safer for appliances that have three hole plugs.
I know the screw in the center of a two hole outlet is often connected
to ground, but
many times an adapter is used to convert a three prong plug to a two prong
plug. I can count on one hand the number of times I've seen a three
prong to two prong adapter that had the grounding lug properly attached
to the ground screw on the outlet. You just
know you're not going to bother to make sure that appliance is grounded,
just like most people don't.
Remember, proper connection to ground, especially in appliances, helps prevent shock and fire. You should replace any two hole outlet with a three hole outlet, unless you are sure the circuit will only receive two prong plugs (Click here to see examples of appliances that use two and three prong plugs). Then you won't need adapters.
Notice that one of the prongs is wider than the other? That means the plug can only go into the outlet one way and its called a "polar plug". Why can it go in only one way?
Appliances can have their internal wiring arranged in the safest possible way for the way the electricity is delivered.
For example: a simple incandescent lamp with a regular 75 watt bulb. Lets say you don't turn off the switch when you start to change the bulb. Its hard to tell which switch setting is off, anyway, if you weren't there when the bulb blew. So when changing the bulb, which would you rather be hot (carrying the electricity)? The threaded metal sleeve that receives the metal thread at the base of the bulb or the small central metal contact way inside the bottom of the receptacle. It isn't that hard to touch the sleeve, or even the metal base of the bulb as it touches the sleeve. But you'd have to try hard to touch the small central metal contact which is way in, at the base of the receptacle.
In other appliances, similar safety measures are taken. And with a three prong plug in a three hole outlet, even more connections to ground are provided.
You can protect your loved ones from this kind of shock with a Ground
Fault Circuit Interrupter (GFCI). You may have seen one in a motel
bathroom. They look like a regular outlet with a couple of buttons. 
Inside they have a way of sensing the flow of relatively small amounts
of electrical current, such as through your body. The worst threat
of such an incident is in locations where someone's body can connect to
ground the best, such as at plumbing fixtures, and wet areas, so kitchens,
baths, basements, garages, and outdoors are the minimum locations you should
put them.
First, there is very low cost, low cost, and moderate cost. However, the first two have disadvantages.
With a guide book, you can check to see if the hot, and neutral holes are reversed, or if the ground, hot, or neutral is disconnected. But it takes a bit of time, and using the guide. Also, if you want to check the GFCI, you can do that at the particular GFCI unit (outlet or circuit breaker) by pushing the test button on the unit. The disadvantage of this is that other outlets protected by a GFCI can't be tested at those outlets.
Well, electric work is a learnable skill, and becomes simple if you read enough or are taught enough, first. But I'll admit, it can make you sweat with apprehension more than most other home repair tasks. You have to learn it right, and after you know it, you have to do it right each and every time. Lets face it, electricity can threaten life, health, and property.
If you're going to investigate and repair, or newly install some wiring, start small, and try to get a more knowledgable friend check your work.
Also, there's no failure in hiring someone else to do it for you. I know some VERY handy people, who won't touch electrical jobs. Just make sure you hire an expert.
Either way, purchase a tester, and check your system out. After
everything is OK, you can sleep well, knowing your loved ones are safe!