An easement by necessity arises when the easement is absolutely necessary to the reasonable use of a piece of property, and the history of the two parcels involved indicates some intention to provide an easement.
The classic scenario for an easement by necessity is the Front Lot - Back Lot, as illustrated below:
Once upon a time, Front Lot and Back Lot were owned by Owner who lived in the House. He sells Front Lot to Al, and then five years later sells Back Lot to Bob. Bob wants to use a dirt path indicated by the red line to access the Road, and seeks an easement by necessity. (It doesn't matter which lot gets sold, just that they do not have a single owner.)
Bob will probably get the easement. In determining if an easement by necessity exists, look for three factors:
1) The two parcels of land were once owned by the same party. (This party could still be one of the two parties involved.)
2) The need for the easement existed at the time the parcels became owned by different parties.
3) The need for the easement is very high, and outweighs the burden imposed on the servient tenement.
In our scenario, criteria #1 has been met, they had the same owner at some point, Owner. #2 is also fulfilled. When the Front Lot was sold to Al, Owner needed the easement to reach the Road. As for #3, the question will be just how difficult it is to travel through the Forbidden Forest. If it's the Forbidden Forest from Harry Potter, where first year students are sent to do chores as punishment, sorry, no easement. But, if it's Fangorn Forest from Lord of the Rings and the Huorns are waking up, you're probably going to get that easement.
When looking at the difficulty of alternate routes, be sure to mention the harm imposed on the servient tenement. Is it an unkempt field that will be relatively unharmed by travel? Is it farmland with crops that would be trampled? Is there already a dirt road there? Do not just say that it would be prohibitively expensive to travel any other way, also say that this cost far outweighs the harm to the servient tenement.
On criteria #3, the most common example is that the Back Lot needs to travel over the Front Lot to be able to get on and off the property. But, imagine if instead of a Road, we had a river, and the Back Lot needed access to the river to water the crops that he grows. Now, the need for the easement is much less and you'll have to get into a discussion about balancing the costs to the two parties. While it's certainly good to get the balancing right, it is not necessary. You will get points for knowing that this is an issue the courts will look at, and for discussing it intelligently.
If an easement by necessity is granted, the servient tenement will get to decide where to locate it. The dominant tenement will have a right to some easement, but not to any particular easement.
Be on the lookout for situations where criteria #2 is not met. If at the time Owner sold the Front Lot to Al, there was a second road running behind the Back Lot, and later on that road became destroyed or otherwise unusable, there will be no easement by necessity. The need for the easement did not exist at the time of the severing of ownership.
Also, keep in mind that an easement by necessity ends if the necessity ever goes away. If the Forbidden Forest gets developed and a new road is laid in connecting the Black Lot to the rest of civilization, the easement will be gone.
Finally, keep an eye out for other easements being tested. The scenario presented above is very common, and people will jump right to the easement by necessity. But, imagine Owner held on to the Back Lot for 20 years before selling to Bob. Could there also be a prescriptive easement? If there is a prescriptive easement, that changes our analysis when a new road is put in, connecting to Back Lot.