You can use a Hydrometer or a refractometer to check the original gravity and final gravity. If using a Hydormeter you can calculate the ABV using those two numbers. There are a number of online calculators if u don't have software like Beersmith or Beertools. If you use a refractometer, there is an extra equation because a hydrometer will not take an exact final gravity because of the presence of alcohol.
Hitting your pre boil, post boil (original gravity) and final gravities on the mark is what we would all like to achieve. Every aspect of your brewing process including ingredients used can affect gravity. What you want is consistent results, which is achievable through practice and good notes and ingredients.
Efficiency can be calculated with any of the software mentioned above and I am sure there are online calculators that will do the math for you as well. What it is doing is calculating the amount of fermentable sugars in the grain bill used for the beer u are making. So if you extracted ever bit of sugar from the grains you are using your efficiency would be 100%, though that is just an example and I have never seen that done before or even possible using standard brewing techniques. Most would probably run in 75-80% range on a good home brew set-up.