Their profile may still be visible, but you cannot send them a direct message
Your previous DM history with them may remain, but new messages will not go through
Their username may appear normally in shared servers, but interactions may feel restricted
You may not be able to add them as a friend
Their status and activity may not update for you
Mentions may not notify them
If you are in the same server, you may still see their messages unless you are also muted, ignored, or otherwise restricted
If they blocked you, reactions and replies to their messages may fail or not appear as expected
A failed friend request or message delivery issue can be a sign, but it is not always definitive
The most reliable sign is that you cannot DM them or add them, even though their account still exists