Ezekiel 9

and the Lord said to him, “Go through the midst of the city, through the midst of Jerusalem, and put a mark on the foreheads of the men who sigh and cry over all the abominations that are done within it.” v.4

Read chapter 9
This chapter can sound pretty familiar to a chapter in Revelation with the mark of the beast in chapter 13 of those that have the mark of the beast during the tribulation time.  Yet the mark spoken of here is not of satan but of the Lord, it's a mark showing who the remanent are that the Lord will spare during the captivity of Israel.  It's a mark that are on specific people who sigh and cry over the abominations that were being done in Jerusalem.  Now this mark wasn't just something like a big X across their forehead that all could see but probably only what the angelic men (v.2) could see so they knew who to kill and who not to kill.  

The word for mark is literally a Tau, the last letter of the Hebrew alphabet. This, in many of the ancient alphabets, and especially in that in use among the Hebrews up to this time, and long retained upon their coins, was in the form of a cross—X or +.  (Ellicot's Commentary)

We don't have marks on us today that we can tell who's a believer and who's not, but what are some signs that cause you to thing they are one way or the other?