Whether it is determined by the status during the original marriage, or at the time of the husband's death: Yevamot 109a
Whether Levirate Marriage of an ineligible woman prevents further Levirate Marriage of an eligible co-wife: Yevamot 111a
If the Woman, while married to the original husband (and while she had live children), swore off benefit from the Brother-in-Law: Yevamot 111b, 112a-b
Above, where she made it clear that her intent was to prevent Levirate Marriage: Yevamot 111b, 112b
If the Woman swore off benefit from the Brother-in-Law, after the death of her husband: Yevamot 111b
If the woman's first husband had a physical defect and she accepted it, but she will not tolerate it in her brother-in-law: Ketuvot 77a
We do not require that the Brother-in-Law be capable of fertilizing the Woman at the time of Levirate Marriage: Yevamot 111b
If a Woman swore off sexual relations from "All Jews": Yevamot 112a-b
If a woman's brother-in-law is repulsive to her: Makkot 23a
Time span after the husband's death, during which levirate marriage is prohibited: Eruvin 47a
At what point in the Sotah process a woman who is married to a Kohen is no longer eligible for levirate marriage, if her husband dies without bearing children: Sotah 2a, 5b-6a
A widow who had been illicitly engaged to the Head Kohen [for he may not marry a widow], when he died: Sotah 6a
A Kohen's wife who was raped, if he then dies and he has a brother who is a "Challal" [a Kohen who has lost his status and is permitted to marry her]: Sotah 6a
General Cases
Levirate Marriage for the wife of a cheresh: Yevamot 112b
Levirate Marriage of a cheresh for a non-cheresh: Yevamot 110a, 110b, 112b
The nature of the bond on the Brother-in-Law for Levirate Marriage for a cheresh: Yevamot 110b-111a
Levirate Marriage for a cheresh: Yevamot 110b, 112b
Above, if she was a non-cheresh when she originally married him: Yevamot 110b
Levirate Marriage of a non-cheresh for a cheresh, where the dead brother was a cheresh: Yevamot 112b
Levirate Marriage of a Woman who is a shoteh: Yevamot 112b
Odd Cases of Co-wives who are up for Levirate Marriage
Where one Woman is a Minor, and the other is an adult: Yevamot 111b
Where both people are Minors: Yevamot 111a-b
Where both people have cheresh status: Yevamot 111b