That's simply a matter of common sense and training. Any host who is running a meeting should preface unmuting the participant by saying something like, "Mary...what do you think about that proposal?" Mary will know that she is being unmuted because how else would she be able to speak and be heard. The host could also say, "Mary...I'm interested in your opinion on that proposal. I'm going to unmute you so we can discuss this."
Having said the above, unless a host is running a training/webinar type meeting, normally all participants in a regular business meeting would already be unmuted because their participation would be expected in a business meeting.