MALCOLM: ...(Macbeth's) fiend-like queen, / Who, as 'tis thought, by self and violent hands / Took off her life
The proof of Lady Macbeth's suicide is saved until the final speech in the play where Malcolm mentions it in passing. The reference to the "violent hands" makes it pretty clear that she
didn't throw herself off any castle battlements, however, and it's definitely the case that he wouldn't be launching an enquiry into how she really died. The truth is that Shakespeare leaves the real cause of Lady Macbeth's death a little bit open - she could have killed herself, either as a result of finally feeling guilt or simply because she knew that Macbeth would lose and she wasn't prepared to be captured by the English; or she could have been killed by Seyton and his minions... either way, and her death wasn't very well covered in the play. She goes from ordering the other Scottish nobility to go home at the end of Act 3 to so anxious she's ready for suicide in Act 5 with NO character arc at all. Why did Shakespeare leave one of literature's biggest turnarounds to happen off-stage? Who knows...