Here are some online suggestions on the topic – feel free to add you own as you go along:

If for any reason, when you ask questions or specifics and the details remain obviously and blatantly lacking, vague or there is any unwillingness to answer, you suspect someone is lying to you for example about age it is normally not a good sign to begin with. If something so small and trivial can unleash all this untruthfulness, would you really want anything to do with this individual in the long run?

TRUST YOUR INTUITION AND INNER VOICE!

• Those who do not respond or seem unwilling, might have something to hide. Think about it… what other logical explanation would there be for them NOT being truthful. (even if they are self-conscious or misrepresented themselves and are now ashamed)
• Inconsistencies, lack of responsiveness, poor follow-up and non-reliability. Empty promises or not answering or calling when they said that they would does not bode well either for the seriousness of their undertaking, commitment and what about this type of treatment in a relationship – would you want that for yourself?
• Not wanting to share a picture might be something or nothing. For some it is a trust issue, but for others they might be hiding or lying and a picture would prove that to you in an instant. Exercise and proceed with utmost caution.
• If for any reason you feel overwhelmed, stalked and/or afraid, intimidated, threatened, or faced with declarations of love and admiration right away or insistence on a personal, face-to-face meeting, WHEN IN DOUBT PASS! Walk away and sever ties!