In the Properties window's > Camera > Display settings you can enable the visibility for limits. It will show you a line and a crosshair for the focus in the 3d viewport:
If you are using cycles you can see the depth of field in rendered view. To set focus precisely is to use an empty and have that empty be the focus object. Moving the empty will move the focus distance:
There is one point where things are in focus, objects closer or further away will be less sharp (or blurrier). Depth of field is defined roughly as the range (distance) in which "things are acceptably in focus", before they are considered blurry.
What is acceptably in focus is determined by the size of the circles of confusion. Watch this video for decent explanation of circles of confusion: https://www.youtube.com/watch?v=Pdq65lEYFOM
The depth of field depends on 3 things:
Lens size - Shorter lenses will have more depth of field. Selective focus is easier to achieve with longer lenses.
F-stop - Smaller values for f-stop (a wider aperture) will make the circles of confusion larger before and after the focus distance, resulting in less depth of field. In blender you can set the f-stop to values that could not be achieved in real life, where it would be impossible to get a lens with an f-stop of 0.01. a larger value for the f-stop will result in larger depth of field.
Focus distance - The closer the objects are to the lens, the less depth of field a lens will have, if you look at images taken with macro lenses you will see that is focus is critical.
Another variable to consider is the sensor size. Using a small sensor requires the use of shorter lenses, making selective focus harder to achieve. Larger sensors will require longer lenses, making it easier to get a narrower depth of field.
There are no tools to visualize the depth of field in blender as far as I know (other than through the camera view). But there are plenty of depth of field charts and calculators online, and a multitude of apps for that.
Depth of Field Utilities
This Add-on allows to visualize the Depth of Field limits (near limit & far limit) directly in the viewport:
Once the Add-on is enabled, select the camera, open up the Properties region of any 3d View (N), scroll all the way down to the bottom and enable the visualization. From there you can fine tune all relevant properties of the selected camera as well as using the 3d Cursor to set the focus distance:
Notice that you can disable the instructions to hit Esc in the Add-on Preferences if needed.