Start with the story
Notice whether details stay consistent over time. A different job title or an old location can have an ordinary explanation, so ask rather than assume. Look for a pattern of contradictions.
Look beyond a perfect photo
A polished picture tells you very little on its own. Review the context around images and whether a profile has a believable history. A reverse image search may help you find a photo's other appearances.
Suggest a short video call
A casual video conversation can help you get to know someone. Repeated excuses may be a reason to slow down, but a refusal by itself is not proof of deception.

