That was the first thought through my mind when they started talking about how many "peace-loving" islamics perished in the collapse. How do we know if they were REALLY there that day or not?
Well, first you check for survivng witnesses. Then check their previous credit card purchases, especially at local travel agencies. Airlines, especially (but not limited to) international flights could provide passenger lists for 9/10 and 9/11 or beyond. Recent bank account closings another way. Moving company contracts, taxi ledgers and storage unit activity are other options to detect recent movements.
There is a story, or unfounded rumor out of Nassau County (NY) that says an Arab-American family warned: "Don't go to 'the city' tomorrow!". That house was supposedly 'vacated' (i.e., empty) after (or on) 9-11 when authorities vsited same, so the story goes. Opinions, rumors and stories may not be "Facts", unless...