Engagement is measured with a survey plus a set of behaviours that show whether the survey is telling the truth. On the survey side, ask the same small set of agreement-scale questions each round so you get a trend rather than a fresh snapshot, and pair it with eNPS: how likely someone is to recommend the company as a place to work, scored 0 to 10, promoters minus detractors. Read participation rate before you read any score, because a result built on 40% participation describes only the people who still believe answering changes something.
On the behavioural side, the useful indicators are voluntary attrition, internal mobility, referral rate and whether people take their time off. These are harder to game than a survey answer. What none of it gives you is a meaningful absolute number: there is no threshold that makes a score good. Compare your score to your own previous round and between teams, because a healthy company average almost always hides one team where the answers are bad.