How do hotels tell guests that check-in time is 10 o'clock?

Just tell the customer directly that the check-in time is 10 o'clock, and then explain why you can check in at 10 o'clock. For example, hello customer, because you did not choose a check-in time for the room you booked, the hotel is now full, but someone will check out later. We will clean it immediately after he checks out, so you need to come back after 10 o'clock. Check in.