What must a researcher know or understand about another researcher to ensure successful research collaboration? What must a researcher know or understand about another researcher to confidently conclude that he or she can become a good partner in collaboration? Specific opinions will be appreciated, e.g. the researcher must understand how his or her potential collaborator's skill set.
The most important thing to know about a potential collaborator is what their expectations are with respect to the collaboration.

For example,


Division of labor - who is responsible for which part of the work? If new tasks come up (as they will), how will the collaborators make decisions as to the division of labor?
Authorship - who will be an author on publications resulting from the collaboration, and in what order? If the collaborators are students, are they planning to involve their advisors in the joint research effort to the point that they'd also be authors? If the are professors, will they involve their students? How will decisions on changes to the author list be made?
Publishing - where will they publish articles resulting from the collaboration? Is one collaborator or another set on a specific venue, or specific level of prestige? Do the collaborators have a similar minimum level of reputation for conferences and journals they're willing to submit to? How do the collaborators feel about pre-publication dissemination of the work, e.g. preprints, presentations?
Timeline - what is the timeline for this research collaboration? Is this a strict timeline, or is it OK if things end up taking much longer? 
Communication - how often will the collaborators check in with each other? Will they meet regularly, or just contact one another to share news (good or bad)?


There are of course no right or wrong answers to these questions; it's agreement that matters. More than any particular skill, compatible expectations are necessary for a successful collaboration.