The therapist only knows what you tell them.  Sure, if the therapist has experience, they could probably surmise some things that you may not have spoken about yet, and hopefully, ask you directly to see if certain things are true for you.  The more you work therapy, the more it will work for you.  The amount of help a therapist can provide is directly proportional to how much work you are willing to do.  This is important to realize because at times, therapists are seen as having some form of magical qualities that will make you ‘better’.    This is not so.  YOU make yourself better by working at making the changes that you want to, and need to, make.  Therapy is useful when you need extra guidance, support or insight into how to make changes and/or address whatever blocks your path.