Continuity mistake: When Wade and Vanessa are standing in front of the Skee-ball, the tattoo behind her right ear disappears, only to reappear when they are handing in their tickets. (00:22:15)
Continuity mistake: In the scene near the end where Ajax has a knife in the side of Deadpool's head and is punching him, later Deadpool pulls the knife out. In a follow up shot there is no hole in the mask where the knife was, but in a later shot the hole is there.
Continuity mistake: At the end the blood around Francis' mouth keeps changing between shots - blood all over his teeth then not, then back.
Continuity mistake: When Wade is lying on the bed talking to Ajax the mouth gag moves around between camera shots - completely missing, then on his neck, then his shoulder, back to completely missing.
Continuity mistake: When Vanessa is trying to remove Deadpool's mask, it goes from having no opening in the back, to having one.
Continuity mistake: Towards the end Vanessa uses her hands to open the cabin and she gets some wounds because of the sword, but when Negasonic helps her the wounds are gone.
Continuity mistake: Deadpool throws Francis at the underside of a vehicle on the highway and items are seen sliding with him, one hitting him. They disappear in the next shot.
Continuity mistake: When Vanessa takes off Wade's mask at the end of the movie, he has a picture of Hugh Jackman's face stapled to his. When Vanessa removes the picture, the staple on Wade's chin is affixed and horizontal. In the next shot, it's hanging down. The third time we see it, it's back to position 1.
Continuity mistake: In the beginning when Deadpool is in the front seat of the cab, the passenger window is rolled half way down, although it seems to change during the scene. But then, when he pulls back his mask, the window is now rolled all the way up.
Continuity mistake: In the bridge fight scene where Colossus and Negasonic Teenage Warhead appear, Deadpool tries to escape Colossus' handcuffs by severing his hand. The hand that he cuts off is his left, but when he jumps off the bridge the severed arm is on his right side.
Continuity mistake: The drawing of Deadpool killing Francis made by Wade during the opening freeze frame is different from the one he draws on the bridge.
Continuity mistake: When Vanessa and Wade are having sex, her tattoo moves to her right breast on Halloween, but is on her left every other time.
Continuity mistake: When the glass chamber holding Vanessa falls off the deck at the end it falls top first, but when the concrete block is lifted off, not only is it the right way up, but it is also at a right angle to the way it fell.
Continuity mistake: The selfie taken by Wade at Christmas changes completely at the bar when Francis looks at it.
Continuity mistake: Just before Wade proposes to Vanessa at Christmas, while she is lying on the bed, she raises her right leg and puts her right foot against Wade's chest. In the next scene, her left leg is now raised and her left foot is against his chest.
Continuity mistake: Towards the end while Deadpool is about to shoot Francis, he puts the gun to his forehead, then Colossus talks to him and now the gun is some inches away.
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.
Answer: Yes that would be how she most likely was able to tell.