Just to the right of the Submit and Cancel buttons. If it's checked, then non-Admin users don't see the edit (including all the cards edited if it's a multi-edit). Better yet, implement add persistent user preferences and GUI for them and let people decide for themselves if they want to see minor edits.
Possibly, mark minor edits in change views (MediaWiki uses a bold m to distinguish them).
+discussed in support tickets+relevant user stories