If you're looking in the web interface, there's a "Group" label on the group's profile page. Other interfaces and apps may or may not show this label.
Don't worry, you did the right thing by removing the @s from the reply! Thank you! 👍
Really, this is a bug, groups ought not to boost posts by non-members, as people are bound to discuss them without wanting to post to them. This is an issue for the group software developers though.