Because Brutus has been acting all nervous and has often left their bed at night, Portia, his good, intelligent and strong wife, suspects that something is up. Brutus tells her that he has not been feeling well, but she can tell that what's bothering him is more than nighttime indigestion. Besides, she has seen him with six or seven men "...who did hide their faces/ Even from darkness." (Act 2, Scene 1). She prods and presses him for the truth about what he's up to, and, finally, he says:
We never get to hear what Brutus tells his wife, but from these words and from what we can feel of the genuine love and respect they have for each other, we can assume that Brutus tells Portia all about the conspiracy to assassinate Caesar.
Later, in Act 2 Scene 4, when Brutus has gone to the senate, Portia is very anxious. You can feel her fear and trepidation in this little exchange with Lucius (who knows nothing of what is planned for this fateful day):
I prithee, boy, run to the senate-house;
Stay not to answer me, but get thee gone:
Why dost thou stay?
To know my errand, madam.
I would have had thee there, and here again,
Ere I can tell thee what thou shouldst do there.
O constancy, be strong upon my side,
Set a huge mountain 'tween my heart and tongue!
I have a man's mind, but a woman's might.
How hard it is for women to keep counsel!
Art thou here yet?
Madam, what should I do?
Run to the Capitol, and nothing else?
And so return to you, and nothing else?
Yes, bring me word, boy, if thy lord look well,
For he went sickly forth: and take good note
What Caesar doth, what suitors press to him.
Hark, boy! what noise is that?
I hear none, madam.
Prithee, listen well;
I heard a bustling rumour, like a fray,
And the wind brings it from the Capitol.
Sooth, madam, I hear nothing.
She can't tell him what she wants him to report back to her about because that would mean letting him in on the conspiracy. She does know that all he has to do is go to the senate and the news will be everywhere. She even thinks that maybe the slightest breeze might bring the shocking truth to her now ultra-aware and fearful ears.