Most people have a dominant eye, but to different degrees, similar to being right or left handed. Some people rely on one eye 75% of the time and 25% on the other. Those people can do the tests you mentioned and figure it out easily. Some other people may have one eye (that's dominant) that they use 51% of the time. Those people are like ambidextrous individuals. I'd say if you're having trouble telling which eye is dominant, it's probably close enough that it won't make an appreciable difference.
What eye do you use when looking through a camera viewfinder or in a telescope?