Many people who continue their prayers properly may have a mark on their forehead. This mark is because of the sajda which makes their forehead darker.
Many people believe it is a blessing mark and some claim that it will be shining in the Day of Judgment, many other things are also attached with this mark.
When the question was raised to Jamia Binoria the mufti responded by explaining that this mark is caused by the sajda and has no significance in religion.
Sheikh Saalih was also asked about this and he also explained that this mark is not a sign that someone is sincere and pious rather many people who only prays occasionally have the mark because their skin is sensitive while many scholars who were righteous and pious doesn’t had the mark.
So rather than judging people by their forehead mark we should follow their acts. If they are obedient to Allah we should be nearer to them if the violate the hukm of Allah we should refrain from them.