Of course, there is that little bell. The voice of God! Ring it and everybody runs to their places and begins doing whatever it is they do to start the Mass. At other times there is someone with an astronomical watch that keeps an eye for the final nanosecond to blink by and sticks his index finger in the air in a “Let’s role” symbol causing the cantor to say, “Please stand now and greet our celebrant with, “Hail Holy Queen.” Let us pray that that never happens again.
Saint Sebastian has a button to press that sounds a buzzer in what in the old days was called the boy’s sacristy. It meant, “Hurry up and get over here! We’re about to start!” There was also an in-house phone system now defunct. It had four buttons on it. One could ring the sacristy, choir loft, rectory, and if there were more locations they have long disappeared. This went a long way in coordinating efforts (and asking last minute questions.) Some parishes have replaced this with walky-talkies. Now that everyone carries every kind of communicative device in their person at all times it seems that even this is unnecessary.
Saint Bernard’s has the equivalent of a rear view mirror just outside the sacristy door so that the priest can see if everyone is ready to go without sticking his head out like someone from an Our Gang epsode before the big show in the barn. Some places have more clever means. St. Ann in Cleveland Hts. has a peep hole built into the intricate grill work in the sanctuary. One goes into a small closet and peeks out at the congregation through a small and inconspicuous hole. One parish I was at had one way glass that almost worked. I wish I could remember where it was but there was a place that had a little door in the sacristy door about eye height. You opened it ala Wizard of Oz (Who rang that bell?) but it was hardly subtle.
Saint Francis in Akron probably has the most practical idea but because it works so well and makes so much sense it is almost uninteresting enough not to mention it. They simply have a peep hole in the door like one finds in a hotel door with the funny magnifying glass in it. It works. Blah. Now if only the door opened the other way . . .