Using words. Same as here on Hacker News.
That is, there is no ack signal.
Reactions are an interesting workaround, where people that need to ack a message can just send one, and the client can collapse repeating ones into a number, not bothering anyone.
But, of course, every client needs to know about that for it to work.
Unfortunately, I work with humans and technology, both of whom are fallible. As such, reminders and prompts are occasionally required. Being able to tell if a message was received, read, and understood is a very useful signal.