The trial heard how Letby murdered newborns by injecting air into their tiny bodies, in some cases shattering their diaphragms, or in one case by pushing a tube down an infant’s throat. She tried to kill two babies by lacing their feeding bags with insulin.
That’s the charge, evidence would be eg camera footage or a witness seeing them put insulin in feeding bags
https://www.independent.co.uk/news/uk/crime/lucy-letby-coded-notes-b2398465.html