By being answered. There are no read receipts, no seen-by list, and no view
counts — nobody is told when you read a message, and you are not told when
they read yours.
On a feed, a reply is the only response a post can receive: no likes, no
reposts, no reactions. The replies under a post are the only tally it carries,
and only once there are some — turning off Show replies in feed collapses them
altogether.
In a direct message or a room, somebody can react with an emoji, which is the
one way you learn a message landed without being answered. A single reaction
is the emoji alone; a number appears beside it only when somebody else picks
the same one.