The latest post by Dick Eastman "Barking Up the Wrong Tree", reminded me about the question a relative asked me when he saw my notebook full of ancestor records; "But how do you know you have the right people?" As Dick pointed out, it isn't enough even to check original records: people in the same town may have identical names.
The answer is in more research, as Mr. Eastman shows - getting the whole picture: not only the person's vital dates, but the names of all associated family - spouse, children, etc. In cases where it is possible to discover the person's employment it can differentiate two otherwise similar people: someone who is a farmer/blacksmith in one census is not likely to have sold the farm and become a pastor or a hat maker in the next.
Beyond checking facts that can be found in contemporary records - original sources, I have found that there is a great variation in the accuracy of secondary sources. It helps to know the reputation of book or writer, and to weigh their information accordingly.
Haste does make waste, and like Dick Eastman, I have lopped off branches from my family tree after finding my research was not thorough enough to discern between my own ancestors and those of someone else. The only thing worse would be to never discover the error, and go blithely on, recording more and more wrong leaves on the branches of my family tree.