Trivia: In the 'happy Lent' scene, Wade is reading the Mario Lopez autobiography. Mario Lopez filmed a promotion with Ryan Reynolds where they pretended the movie was going to be toned down to a PG-13, which would have infuriated fans, before revealing the true R rating.
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: 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: At the end the blood around Francis' mouth keeps changing between shots - blood all over his teeth then not, then back.
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 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: The selfie taken by Wade at Christmas changes completely at the bar when Francis looks at it.
Continuity mistake: When Vanessa is about to take off Deadpool's mask her left hand changes position between shots.
Continuity mistake: When Deadpool causes Ajax to crash, the debris around him on the ground disappears between shots.
Answer: Yes that would be how she most likely was able to tell.