In Breaking Bad, season 3, after Walt kills the two dealers who murdered Tomas, he drives out into the desert where he meets Mike who then escorts him back to Gus's car. He suggests to Gus that he has two options: option "A" being to kill Walt and Jesse, and option "B" being to let it go and keep Walt cooking, meanwhile forgetting Jesse ever existed. It seems to me that Gus chooses option B, when he says "You'll need a new lab assistant [...] this time I choose".
However, even though the agreement seems to materialise with Gale back on the team, Walt admits to knowing about Gus's intentions (revealed to Gale a few scenes earlier) when visiting Laser Tag with Saul. He seems far beyond suspicious about it. How can he be so sure?