How can we tell who has or hasn’t taken the engagement survey?

You can't see who individually has or hasn't taken the engagement survey, because it's anonymous. What you can see is your overall participation rate.

The engagement survey is always anonymous, on purpose. When managers can't tell who did or didn't respond, employees feel safe giving honest feedback. So Retain never shows who has or hasn't taken it.

What you can track is your participation rate, the percentage of eligible employees who have responded. See Find Survey Participation Rates.

To get more people to respond, see Boost Engagement Survey Participation.