Revealing mistake: In the scene where Captain Love and Antonio Banderas (as Zorro) meet in the hallway of the hacienda, two guards rush down the hallway and drop their rifles and swords. Zorro asks them to bend over and then proceed to kick/nudge them out the window. If you watch carefully, you'll notice that the guard on the right (furthest from Zorro) reacts to the kick too early. (01:33:30)

The Mask of Zorro (1998)
Plot summary
Directed by: Martin Campbell
Starring: Anthony Hopkins, Antonio Banderas, Catherine Zeta-Jones
Diego dela Vega isthe Zorro, the people's hero.He is arresting when he is discovering by his enemy Montero.
Years pass Montero retuns.Diego de la Vega escapes of prison.He goes to kill Montero.He meets Alejandro, a man than also wants to kill Montero.They kill Montero together.
Three Finger Jack: Hey, basura! Up here! Well ain't you a sight for sore eyes. Full grown vultures walking around like real folk. Welcome! Welcome to Hell's outhouse! Now they call us the disappeared once but as you can see we ain't exactly disappeared. We've just been a little hard to find.
Captain Love: You. I know you.
Three Finger Jack: You bet you do, peckerwood! I'm the legendary Three Finger Jack and you are a bunch of murdering trash dressed up in fancy sweet smelling doodads!
Captain Love: Ignore him gentlemen. He's a common thief.
Three Finger Jack: Hah! As common as they come, but I ain't nothing to you "gentlemen." I steal gold. I steal money, but you... You steal people's lives. So damn you, and damn the whores that brung you!
Trivia: Antonio Banderas was the first authentic Spanish actor (he was born in Spain) who played the Zorro character. All of the previous actors in old movies or TV series only had the Spanish appearance or ancestry.





Answer: He recognized that Bernardo was really Diego de la Vega and he realized that the young Zorro he had seen earlier could not have been Don Diego. He then deduced that the new Zorro looked similar to Don Alejandro (and was probably warned by Captain Love that Don Alejandro was hiding someting) and made the decision.