As with real estate, location, location, location! If he came from a small town in Ontario, for example, the Thomas Lynch up in Manitoba (for example) probably isn't him. Look at maps, mark out where you have found him for sure, and see what is nearby. Of course this gets complicated if he did a lot of migrating, but it's a good place to start. (No pun intended.) Also, build out his family--if he lived next door to his brother-in-law, and then his brother-in-law moved to another place, but a Thomas Lynch still lived near him, that's probably your guy.