When should you "lower your gaurd"

Or should you even lower it? When a woman (or a man) has been hurt in the past, it may be difficult for them to enter in the dating scene again. As much as they try not to hold a grude, or compare people's actions to those of the former person, subconciously we all do. We all look for a red flag that will comfirm that indeed that person is a fraud/liar/jerk/asshole. We become pessimist. But when shall we open our hearts to the new person in our lives?? How will you know it's right??