We see, ask for, and contribute to formal and informal reference checks often (“Hey, do you know x person? How are they to work with?”). And, while most folks who worked closely with a person can provide useful feedback, I’ve observed the following for the highest signal-to-noise ratio:
1) How well they communicate and hold a room -> their manager’s manager, an executive they reported into, or their manager (in that order).
2) Their ability to both lead and be a good colleague/teammate -> folks on cross-functional teams with no reporting relationship.
3) How good they are at their job -> peers, followed by the manager.
There are exceptions to these observations – typically folks up the reporting chain who manage to stay abreast of the detail. But, that aside, hearing strong positive feedback from a peer with close working experience is among the strongest positive indicators of a person’s ability to contribute on the job.