YSK all iPhones have IMEI numbers in both their settings and printed on the sim card tray. You can run this number through a plethora of websites to see if the phones locked, blacklisted, has Find my iPhone on, or is iCloud locked.
from /r/YouShouldKnow
YSK所有的iPhone在其設置中都有IMEI號碼，並印在SIM卡托盤上。你可以通過大量的網站運行這個號碼，看看手機是否被鎖定，是否被列入黑名單，是否開啓了查找我的iPhone，或者是否被iCloud鎖定。