Tactfully. The best way is to talk ahead of time to any doctor or student and figure out a way to give messages to each other that the patient should not hear.
Always keep a pen handy and if you have a sticky pad too, that helps. When my assistants need to give me a message they write it on a sticky pad and put it behind the patient and say, "Doctor, you have a message." When I hear that, I know it means I have to stop and look at the message cause they don't say it unless it's something that needs my attention and can't be said in front of the patient. If it's something that needs to be discussed away from the patient, the assistant will write "Message" and that means I need to excuse myself away from the patient and the assistant will follow shortly and tell me what is going on.
If my assistants hands are occupied and they see me about to make a mistake, they will say something like, "Doctor, are we working on tooth 4 or tooth 6 today?" Most patients won't really think anything much about this, but it usually means I need to check what tooth I'm about to prepare. Only had this happen once or twice...but it's always good to have keen assistants.