In Trello, I see a list of Notifications on the right with a red alarm "bell" to signify I have new notifications ( @comments to me, etc.)
But, I can't tell which of them I've already viewed.
One possible solution:
A card that has a Notification since I last looked at that card will show the notifcation (red alarm bell) icon.
But, I see no way to search on those cards.
Best Answer
When you click on the red alarm "bell" you should see a list of new notifications. Look for the darker side bar on the left side of each notice in the list. Trello clears this bar when you close the notification list.
Click the bell again to see the difference.
Notes: