By looking for them. Seriously. This is one of the stupidest posts I have seen by you. You are better than this.
I would start by looking where they should be, their rooms. If they are not in there I would not shoot until I knew where they were. If I don't know they are behind me... Where they are... I simply would not shoot. There is a greater chance of me shooting them than an intruder if I follow any other plan.
Let's say one of my daughters was out staying the night elsewhere. I don't know that she hasn't come back... I wouldn't shoot until I had identified who I was shooting at. Yeah, it may put me in more danger, but I would not shoot without knowing I was not shooting at one of my own family. There is still a greater chance that I would wind up shooting my family member than an intruder at that point.
These are things I learned in home defense classes. They weren't even expensive.