Security expert Bill Stanton spoke to INSIDE EDITION from New York City's Grand Central Station, where there is heightened security in the wake of the bombings.
"[People] need to be aware of their surroundings," Stanton said. "They need to look out for something that's not in the usual."
He explained: "Look for people that may be nervous, that may have a bag, looking left and right, drop that bag and walk away. You see something like that it's time to use that cliche: you see something, say something. Go to a law official."
Missing a train or a flight is worth flagging the potential danger, he said.
"You miss that train and you may save someone's life," he said.
Stanton added that any time you find yourself in a railroad terminal, an airport, a movie theater or a mall, figure out how to escape in an emergency by looking for nearby exits. Look for multiple exits so that you have options, depending on where an attack occurs.
So how real is the threat here in the U.S.?
Terrorism expert Robert Strang told INSIDE EDITION that ISIS is focused on Europe, but a similar attack could happen here at any time.
"We try in the U.S. to keep a little bit more information on people who are coming into our country," he said. "But to be honest we don't know everyone who is in our country so we also have this potential problem as well."