I haven't been successful in absolutely determining matches in cases such as you cite. HOWEVER, because of general family migration patterns, I can often figure that there is probably a match amongst this line (or another) due to looking at surname, location AND time period.
For example, I have lots of colonial lines. I've been able to trace from say eastern NC up to Virginia or Maryland. I've paid attention to other names in the censuses (family groups tended to migrate together, and often intermarried). You may look at the surnames and the years and the location and find that's an indication that some generation or two further up your tree (or theirs) may be where your lines converged.
Another thing that has helped me is to continue to flesh out the cousins and the siblings of my ancestors. That links me up to more surnames and locations.
AND, when I'm looking at the trees, I ALWAYS look at the PEDIGREE to go upward. It's useless to compare the family view until you've gotten back to the common place and time...