What are the laws that the Psi Corps have about a Telepath scanning the minds of people?
Permission is required, but that doesn't mean it's always acquired.
The rules are never comprehensively laid out, but they're referenced a few different ways. For example, in "A Race Through Dark Places" (s02e08):
Bester: I think they'll do as we ask.
Talia: You seem awfully confident.
Bester: I am.
Talia: Did you scan them?
Bester: Ms. Winters, you know the rules against scanning normals without their permission.
So, we're told there's a rule against scanning normals. But when Bester answers in this way, it is hinted that this answer was misdirection; that he did indeed scan them. I seem to recall other interactions, especially with Garibaldi, where Bester clearly implied that he considered himself above the rules about scanning. (And, Bester being Bester, there are also times where he clearly used the implication of it to rile someone when he'd apparently done nothing).
And in "Eyes" (s01e16), we see an example where the rules get re-interpreted:
Ivanova: Why is Psi Corps in this routine? Regulations prohibit the use of telepaths.
Col. Ben Zayn: There are new regulations, Ivanova. All staff must submit to telepathic scan as part of an investigation. Command staff as well.
Which led to Sinclair clarifying:
Sinclair: I also checked Earthforce law and the opinion of the Senate Military Bureau. They state that scans are allowed only to determine the truth of charges.
and, thereafter, being subjected to a tribunal where (legally) a telepath was used to verify the truth of his statements under oath. As a result of that Tribunal, Col. Ben Zayn left without completing his threat of wider scanning, but it's implied or stated that he got away with a lot of scanning before someone stood up to him.
And in "The Quality of Mercy" (s01e21) we're also told that it's inadmissible as evidence in criminal matters:
Talia: Scanning like that violates the right of due process. Anything the telepath sees while inside a defendant's mind is inadmissible.