1. I think that most times, the dominant trait will be most common. Because the phenotype is indeed dominant, it most likely will be more common just because of the way it is made.
2. A phenotype is determined by past ancestors. For instance, in my family, both my parents have unattached earlobes, while my grandmother on my dad's side had attached. both my grandparents on my mom's side have unattached earlobes. My sibling's earlobes are determined by my parents, grandparents etc.
3. No, it is not possible to determine the genotype of a person showing a dominant phenotype. You cannot tell how much of one trait comes from your parents, it can be a 1:1 ratio, 3:1 ratio, etc.