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在其设置中都有IMEI号码，并印在SIM卡托盘上。你可以通过大量的网站运行这个号码，看看手机是否被锁定，是否被列入黑名单，是否开启了查找我的iPhone，或者是否被iCloud锁定。