When I call places, I of course expect my actors to give me a "thank you, places" so that I know they've heard me. After that it is their responsibility to get into places, or let an ASM know if there is a problem. Before GO, I get a confirmation from ASMs that everybody in the first few moments of the show is present. This gets kind of tricky when you're working in a black-box or other flexible space like ours, so I usually have to improvise show-by-show. This usually just means that I have an ASM run to any entrance that they cannot see, and then report back to me. Sometimes, I have to rely on the House Manager because depending on how the show is staged, we frequently have actors enter from the doors leading to the lobby. So, typically I will call places, but the actors will wait in the stairwell until she closes the house and informs me that she will let the actors in. HOWEVER, the last show I required my lead actress to be seen in an upstairs window of the set, which was accessed via the upstairs balcony of our mainstage. This was the very first moment after the house went out, so I had to be sure she was there. I ended up having her reach into the booth from the hallway and tap me on the shoulder as she was going her place (she had to walk past anyway). It worked well to assure me that we were ready to begin the show, and saved me from running back and forth.
SO, I guess the point of my little tangent here, is that I have a general procedure for going to Places, but it varies depending on the needs of the show, the size of the company, and how busy my ASMs are before the show.