What suspicion does Banquo voice in "Macbeth"? What does he say that lets us know what he suspects?
The night Macbeth plans to kill Duncan, he and Banquo speak briefly. Macbeth says, "If you shall cleave to my consent, when ’tis, / It shall make honor for you" (2.1.24-25). In other words, Macbeth tells Banquo that if he will act as Macbeth would wish when the time comes, it could increase his honor. The noble Banquo responds,
"So I lose none /
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counselled" (2.1.25-28)
He means that he is unwilling to do anything to elevate his station or improve his situation that might ultimately reduce how honorable he is. Banquo doesn't want to do anything that will saddle his conscience with guilt. However, why would he say such a thing unless he suspected that Macbeth could ask him to do something untoward? He must be suspicious of Macbeth or else it probably wouldn't occur to him to present this caveat.
Then, after Duncan's murder and Macbeth is crowned king, Banquo says, as though speaking to Macbeth,
"Thou hast it now: king, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou played’st most foully for ’t" (3.1.1-3)
We know he's speaking as though he were addressing Macbeth because he references Macbeth's acquisition of the titles "king" and "Cawdor." When Banquo says that he's afraid Macbeth "played foully" for all that he's achieved, he explicitly expresses his suspicion that Macbeth has done something bad in order to rise to the position the Weird Sisters predicted.
At the beginning of Act 3, Banquo, in a brief soliloquy says, "Thou hast it now - King, Cawdor, Glamis, all, As the Weird Women promised; and I fear Thou play'dst most foully for't." He is saying that Macbeth (Thou) has seen all the witches' prophecies come true, but he thinks that Macbeth played foul and committed murder (play'dst most foully) to make the prophecies come about. Even back in Act 2, sc. 1, before the killing of Duncan, Macbeth tells Banquo that he wants to talk with him sometime on the subject of the witches' prophecies and he goes on to hint to Banquo that listening to him, Macbeth, could be good in the long run for Banquo. Banquo's response is that he'll listen to what Macbeth has to say as long as he can keep his conscience clear and show no disloyalty to the king. Macbeth probably knew from that remark that Banquo would not go along with the murder or any knowledge of the murder.
In Act III, scene 1, Banquo says "Thou has it now: king, Cawdor, Glamis, all, as the weird women promised, and I fear thou play'dst most foully for 't."
Banquo suspects that Macbeth had a hand in it. He was told by the witches that he would become Cawdor and King, so his sudden acceptance of the throne makes Banquo question how he got there so quickly. He wants to remain loyal to Macbeth (as the king) but he does not trust him at all.