In The Avengers:
Natasha and Loki are talking while he's trapped in the cage on the ship, they speak for a while about her. Then he tells her he's going to get Hawkeye to kill her slowly and painfully before killing him. At this point she says something along the lines of "So you're going to use the Hulk to destroy the ship?"
How does she figure this out?
In the movie:
The 'evil Hawkeye' states that he needs a big distraction. So Loki makes himself publicly noticed in a big way, and is captured by the Avengers pretty easily, which raises some suspicions.
In the scene:
Black Widow calls Loki a "monster", and Loki responds that there already is a "monster" on this ship, alluding to Bruce Banner. From this she is able to deduce that Loki's goal is to awaken the Hulk in Dr. Banner (a big distraction).
She came to this conclusion because:
Not many know that Dr. Banner is the Hulk. If Bruce were to turn into the Hulk, it would be a disaster, answering why Loki let himself get captured so easily.
In addition to acknowledging that Loki evidently had fore-knowledge of Bruce Banner being the Hulk as described above, I would suggest that if Black Widow has any kind of "super power" (if you assume that Hawkeye also has a "super-power" that is decidedly human, i.e. perfect archery skills, or maybe even a gun-sliger's "sixth sense" regarding aim and wind, etc), besides her assassin training / perfect martial arts, her powers would include an uncanny ability to extract information.
This is clearly shown in the beginning of the movie where the bad guys think they are interrogating Black Widow but in fact she is playing them as they spill the beans on almost every aspect of their plans. Her skill in this regard would be nearly unparalleled, using her brilliant mind and emotional manipulation, as well as misperception regarding "feminine frailty," to pull even the god-like Loki into spilling the beans.
In this regard, comparing the dialogue to interrogation techniques, she starts by a) pretending to befriend Loki, b) pretends to think she is coming to him for a deal, c) pretends to expose her emotional attachment to Hawkeye (despite the fact that her training included the near nullification of all emotion into a cold, killing machine), and d) leads an extensive dialogue aimed at drawing Loki to express his plans by playing on his obvious, gigantic ego.
One would think the conversation would have gone on and on as Black Widow would slowly have inserted one emotional play after another into the black-box of Loki's mind until a response came that gave her the information she sought.
These same techniques are used by psychologists in certain kinds of therapy when helping a client figure out motivations buried so deep they are mostly subconscious.
A probing question or comment is offered, and the client's response then directs the next question until a pattern of thought, or an effective pathway through the maze of the client's mind, is identified.
Loki, like sooooo many villains, has an enormous ego that when plucked properly practically kick-starts his need to be perceived as great by way of his self-proclaimed genius. I.e. open mouth, spill beans. Black Widow is an expert at this process.
I assumed it was because Loki had already hinted at the Hulk so much
especially while talking to Fury ('the mindless beast', etc.)
