I don't think they'll be looking stereotypically Muslim (whatever that might be) but I don't think shoe bomber Richard Reid and underwear bomber Umar Farouk Abdulmutallab looked like businessmen in three-piece suits. Reid wasn't clean-shaven at all.
Focusing on garb instead of behavior is probably the wrong way for security to find potential terrorists.
These are the real red flags:
"...The near miss aboard the Northwest/Delta flight highlights the difficulty in setting screening in the right places to catch would-be terrorists.
Britain's denial of entry to Abdulmutallab may in itself not have required the U.S. to be informed, British officials said. But even without that clue, Abdulmutallab's
recent stay in Yemen, combined with
his father's warning and the fact that
he paid cash for a one-way ticket and
didn't check any luggage, should have been sufficient to set off alarm bells. Or at least a more thorough search before he climbed into seat 19A aboard Flight 253...."
Why Was Abdulmutallab Banned in Britain but Not the U.S.? - TIME