How do we see in this verse that Jesus already knew Simon and Andrew from before (see John 1:35-43)?
That Jesus knew them is apparent also from how Mark phrases the story. It does not say that Jesus saw fishermen who are called Simon and Andrew
; rather, it says that he saw Simon and Andrew.
Their names are known by him. The concluding words of Mark 1:16 explain that they were fishermen and that makes it clear why Jesus finds them here.1
16 Comme il passait le long de la mer de Galilée, il vit Simon et André, frère de Simon, qui jetaient un filet dans la mer; car ils étaient pêcheurs.