Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
/* Sysop decrier/detector (rights group displayer), version [0.2.1]
Originally from http://en.wikipedia.org/wiki/User:Splarka/sysopdectector.js

Notes:
* Fixed this up to use the new API fun stuffs.
* Shows all groups now
* nstab-user isn't available in all skins, all skins have at least one h1 or h2 I believe.
* heading given class="detected-userrights-heading" and text in span class="detected-userrights"

Options:
* Now supports an option system.
* By setting a datalet true it shows in the header instead of the title.
* Note, all are assumed false if omitted. All but 'groups' are assumed false if undefined.
var showUserGroupSettings = {
	'IP':true,
	'unregistered':true,
	'registered':true,
	'groups': true,
	'editcount':true,
	'regdate':true,
	'blocked':true
};
*/

function showUserGroups() {
	var url = mw.config.get( 'wgServer' )
		+ mw.config.get( 'wgScriptPath' )
		+ '/api.php?action=query&format=json&callback=showUserGroupsCB\
&maxage=3600&smaxage=3600&usprop=blockinfo|groups|editcount|registration\
&list=users&ususers='
		+ encodeURIComponent( mw.config.get( 'wgTitle' ) );
	mw.loader.load( url );
}

function showUserGroupsCB( obj ) {
	var show = window.showUserGroupSettings || false;
	var user = obj.query && obj.query.users;
	if ( !user || !user[0] ) {
		return;
	}
	user = user[0];
	var someHeading = document.getElementsByTagName( 'h1' )[0] || document.getElementsByTagName( 'h2' )[0];
	if ( !someHeading ) {
		return;
	}

	var span = document.createElement( 'span' );
	var title = 'User:' + user.name + ' ';
	var text = ' ';

	if ( user.invalid === '' ) {
		if ( show && show.IP ) {
			text += '[IP] ';
		} else {
			title += '[invalid or IP username] ';
		}
	} else if ( user.missing === '' ) {
		if ( show && show.unregistered ) {
			text += '[doesn\'t exist] ';
		} else {
			title += '[not a registered name] ';
		}
	} else {
		if ( show && show.registered ) {
			text += '[exists] ';
		} else {
			title += '[username registered] ';
		}
		if ( user.groups ) {
			if ( show && show.groups || !show ) {
				text += '[' + user.groups + '] ';
			} else {
				title += '[' + user.groups + '] ';
			}
		}
		if ( user.editcount ) {
			if ( show && show.editcount ) {
				text += '[' + user.editcount + ' edits] ';
			} else {
				title += '[' + user.editcount + ' edits] ';
			}
		}
		if ( user.registration ) {
			if ( show && show.regdate ) {
				text += '[created: ' + user.registration.split( 'T' )[0] + '] ';
			} else {
				title += '[created: ' + user.registration + '] ';
			}
		}
		if ( user.blockedby ) {
			if ( show && show.blocked ) {
				text += '[blocked] ';
			} else {
				text += '[currently blocked] ';
			}
		}
	}

	span.setAttribute( 'class', 'detected-userrights' );
	span.appendChild( document.createTextNode( text ) );
	someHeading.appendChild( span );
	someHeading.setAttribute( 'title', title );
	someHeading.className += ' detected-userrights-heading';
}

if ( ( mw.config.get( 'wgNamespaceNumber' ) === 2 || mw.config.get( 'wgNamespaceNumber' ) === 3 ) &&
	mw.config.get( 'wgTitle' ).indexOf( '/' ) === -1 &&
	( mw.config.get( 'wgAction' ) !== 'edit' || mw.config.get( 'wgAction' ) !== 'submit' ) ) {
		$( showUserGroups );
}