How can you see all watchers on a trello card. You can do this on JIRA...

[Robert Rawlings]

The intention is to have one responsible person on a trello card, with watchers who are either PM's or have vested interests

[Stef]

Based on the definition of a watcher, it sounds similar to being a member of a Trello card. Once you add members, you'll see their avatars right below the card title and under the label MEMBERS. Is this what you're referring to? 

[Robert Rawlings]

If we add multiple members to a trello task,  how would we then indicate who is primarily responsible for this task. If I want to view who is late on tasks,  I want to see the responsible person,  not everyone else. How can you achieve this on Trello? 

On JIRA,  I assign 1 person as the person who is responsible,  and then allocate watchers who have a vested interest. I can see who all the watchers are and check that everyone  that should be kept in the loop will be (by virtue of them being a watcher) .

[Garrett]

Currently, we don't have a direct way to specify which user is responsible for a task out of all members of a card. For now, I'd suggest adding an @mention of the designated user to the card's description.

If you'd prefer to have the assigned user be visible on the front of the card, you can go the Custom Fields route. With this you can create a dropdown list or text field with your users like so: