So if Sally isn't sure she'll qualify as a dependent for 2020 (she may already have five months of attendance, if Zoom classes count), should she hold on to the check, or cash it?  If she knows she will qualify as a dependent, should she send it back now?

This is when you have The Conversation with your client.  "If you ask me if she should have received the check, my answer is that she did not qualify.  If you ask me what happens if she keeps the money, my answer is that it's highly unlikely that the Justice Department will sue to get it back."