Continuity mistake: Just before Wade escapes from Colossus on the bridge they are more in the middle of the street, then suddenly they are beside the border.
Continuity mistake: When Deadpool causes Ajax to crash, the debris around him on the ground disappears between shots.
Continuity mistake: When Wade and Weasel decide on the name "Deadpool," Wade is wearing gloves, but in the close-up, his hand is bare (and un-scarred).
Continuity mistake: When Vanessa is about to take off Deadpool's mask her left hand changes position between shots.
Continuity mistake: Between Wade stabbing Ajax to the bridge barrier and him talking to Colossus, the sun changes every shot.
Visible crew/equipment: In the opening fight in the vehicles, Wade looks at the SUV pulling up alongside. As it does, the camera recording from his point of view is reflected on the door.
Continuity mistake: When Deadpool comes back to the duplex after cutting off his own hand, he grabs a red towel off a Chevy Citation parked outside. When he enters and flops onto the floor, the towel is yellow.
Answer: Yes that would be how she most likely was able to tell.