In Act II, what evidence showed that Brutus and Caesar could have been influenced by flattery?
There is very clear evidence of Brutus and Caesar being flattered and giving in to flattery in Act II of this play. Flattery and speech used to manipulate other characters is a key feature of this play and we see this throughout the play.
In Act II scene 1, for example, when Cassius and the other conspirators arrive at Brutus' house for their meeting to plot the death of Caesar, Cassius greets Brutus with the following words:
...and no man here
But honours you; and every one doth wish
You had but that opinion of yourself
Which every noble Roman bears of you.
This is an extension of the "seduction scene" in Act I scene 2, where Cassius flattered Brutus mercilessly. It also supports Cassius' strategy of leaving documents around in Brutus' house, supposedly written by other noble Romans who say how much they admire and respect Brutus. I can't help but think that Brutus at this stage is pretty much a tool in the hands of Cassius - he is manipulated to join the plot.
Of course, in Act II scene ii, Caesar is likewise manipulated by Decius, who is very sure of his ability to persuade Caesar to do his will, as he shares with the other conspirators in Act II scene i. After Calphurnia has apparently succeeded in persuading Caesar to stay at home and not go to the Capitol, Decius re-interprets her dream in a "good" way and then subtly hints that if Caesar does not go he will lose his chance to gain the crown, saying that the Senate will think him weak:
If Caesar hide himself, shall they not whisper
"Lo, Caesar is afraid"?
Pardon me, Caesar, for my dear dear love
To your proceeding bids me tell you this,
And reason to my love is liable.
Note how wily Decius is being here - he says it is only his "dear dear love" which causes him to suggest such thoughts to Caesar, but also taps into Caesar's desire to receive the crown and gain more power.
So both characters are very definitely persuaded and flattered in Act II by others for their own purposes and motives.