You don't, you never can be absolutely sure of anything. You make your best guess with the information you've got, and go with it. Don't make yourself nuts by rehashing your analysis over and over with the same information, trying to convince yourself you must be right.
Only rethink it when new information comes your way. Remember new information doesn't automatically change how you feel. And sometimes even when it changes your mind, it's better to keep on, keeping on, rather than make a disaster trying to change course.
Sorry to not be more reassuring, but that's a lesson I learned the hard way... several times.
