As far as I can tell there's no built-in way to view the members list which reports who is an owner/admin, if you're logged in as a regular team member. I would really like this feature, but in leiu of that I managed to simply browse to the URL of the team's user page (<team_id>/settings/team/<team_id>/users). I'm assuming it's not intended for this to be accessible by non-admins because I was able to right click on a user and select the "Set as admin" option. This of course fails with a small error that pops up in the bottom left corner (web interface), but the front end still updates to show the target user as admin, without checking if the action should succeed.