I generally just have a pretty good instinct for it. If there's a double character, like double 'o's then the one with the single version could work. usually taking the last letter off the domain can work. If it its an 'ei' domain then registering the 'ie' domain could work.
It really varies from domain to domain.
You can also use the linkpop at startname to check google searches for each typo
this is far from being an area of expertise for me but words that look alike are probably useful in some way. The amount of times ive seen friends and family type in www.goggle.com rather than www.google.com is beyond belief.
Studies have been shown that when you read something, you comprehend the first and last letters of a word, the rest you dont think about where they are, you know what they are but your mind just puts them logically where you think it makes a word, but its not necesarily what it actually says.
Thats why in a sense in England the clothing conpany French Connection got so popular, they abbreviated their name to FCUK so everyone who saw that on a big banner while walking down the street would give it a second look cause they saw the first and last letters and didnt conprehend immediately what order the middle two letters were placed in.
ill expand more if someone doesnt understand.
btw, as i said, i have no expertise in this field but its just an assumption.
Mateja has a great psychological thought-process approach to this that seems to be pretty accurate. I definitely find myself typing typos like those all the time, while online.