If you know he logged in, it cancelled. If you’re not sure he logged in, then check his stats to see if he’s done any attacks or made any donations since the start of the season. Check his base for cleared tombstones or trees. (Although that isn’t reliable unless he likes a neat base.)
How can I know if the leader's automatic change system was canceled because the leader logged in?
No league shield.
Traps, ITs, Xbows, EA dead where applicable.
tomb stones (though they do not disapear automatically)
full collectors (or close to ful)
wait another 40 days.. If the leader was inactive, he will not be leader by then.