As it stands, it is no possible to add indicators on Cards View, like for example, when there are attachments. But this is actually a really good feedback so I’m going to move this post to #productfeedback category so other users can vote for it. I have also changed the title of your post to make it more discoverable for other users that are looking for this same feature to be implemented. I hope that is OK:grinning:
Also, there are other similar requests that you might be interested in. Please, feel free to add your vote if you like.