Dial bore gages are used as comparators. You will be comparing the bore of your work against the bore of a master, such as a ring gage or a preset micrometer. If your work piece has to be .375" for example, place your bore gage head into a .375" ring gage and set the dial to zero. Then place the bore gage head into your work piece and you will be able to see how close to zero your bore is. The graduations on the dial, be they .0001" or .0005" (or metric) will indicate the difference between your work piece and the ring gage.
Bore gages typically have 2 point measurement. The fixed—but interchangeable—range contact is opposite the movable measuring plunger. This arrangement allows you to check for irregularities in the bore. You'll easily see if you're out of round, or if you've created a taper.
There exists a split head style of bore gages, Diatest and Mitutoyo being notable. While these are also used as comparators, they can not be used to check the geometric shape of the bore because they don't use two point contact. If that's something you need to check, then you will have to eschew this style. They have the advantage of fewer moving parts and can be less prone to mishandling.
Dial or digital bore gages can not measure to the very bottom of a blind hole because of their design. However, Mitutoyo makes a bore gage which can get within 2.5 mm of the bottom. The other styles are best used on through holes.
Since direct measurements are never taken with a bore gage, there is little point in having the gage calibrated. What is important is that the indicator hand moves freely and the gage doesn't stick; that there is no excessive play in the movable anvil at the measuring head; and that the fixed or interchangeable anvils (there may be one or more) are not worn flat. If any of these conditions exist, then the gage needs servicing. The dial indicator which is attached to the measuring head must, of course, be accurate and this indicator can be calibrated just like any other dial indicator. (See this web site for information on dial indicators.) Please note: if you need to take direct measurements of the bore then you'll want to use an internal micrometer or a bore micrometer such as an Intrimik or ingage.
Bore gages are available from many manufacturers with a great range of prices which are a reflection of the tool's durability and workmanship. Gages with carbide plungers and points will of necessity cost more but last longer. On the other hand, steel plungers and points are cheaper to buy and replace. As with other gages, if the tool will only be rarely used there is not much sense in investing in high-end equipment. All bore gages are accurate comparators when new, whereas the better brands are likely to last longer. (We are still repairing Compac dial bore gages that were made in the 1950's.)
An important consideration when buying a dial bore gage: all bore gages use a series of extensions and contacts; these parts eventually wear down or get lost and you will need replacements. Be sure to buy a bore gage that has replacement contacts and extensions readily available otherwise you will be left holding a useless piece of equipment.
The measuring head and the indicator portion can usually be purchased separately if one or the other becomes damaged. Replaceable anvils and other repair parts are generally available except for the inexpensive models such as Teclock and other generic Asian imports.