What's the best way to look for a lost parent?

Private detectives are usually for people who don't know either of their parents, and who don't know anyone who does know them. It sounds like all you need to do is get your mother to tell you. Really put pressure on her. Don't take no for an answer. You could threaten to cut her out of your life entirely if she won't tell you. Don't let her see her grandchildren. You could even embarr her about it in front of her friends, coworkers, and church.