It really depends how far down the line you want to go.
The easiest way would probably create the list of common (typing/spelling) mistakes yourself and check that against the answer..
Otherwise it would consist of checking if the right answer is somewhere in there, by comparing the length of the answer, each letter compared to the right answer, etcetera..
If you want to go that way you should look at the system-expressions for strings/text..
I guess you are looking for some kind of implementation of the LevensteinDistance-algorithm..