You know you have unread messages and unseen activity in a channel because its name appears bold. If you favorite the channel, it stays visible in your list.
You also receive notifications when someone @mentions you or replies to a conversation you're in, so that's another way you know a channel is active.
If you're worried about knowing whether a conversation is going on that needs your attention, rest assured: All your notifications appear in your Activity feed, so you'll never miss a thing. You get a notification whenever someone:
@mentions you (you also see your @mentions in the conversation).
@mentions teams and channels you belong to.
Replies to conversations you're part of.
Replies to your replies.
Notes: Another way to check your notifications is to use the command box at the top of Teams.
Type /unread to see your unread channel notifications.
Type /mentions to see all your @mentions.
Only channels you favorite are visible from your team list. A bold channel is an active channel that has unread messages. If a channel shows a number, that shows you how many times you’ve been @mentioned.
For @teams mentions, you'll see this team icon in your activity feed.
You'll also receive notifications in your activity feed when someone @mentions you or replies to a conversation you're in.