This is definitely the biggest clue.
But, I have to say that I’ve never snooped an employee’s LinkedIn. As a manager and leader, you should always be striving to treat your team the absolute best that you can. That includes comp, challenging and rewarding work, the way you behave as a manager, a healthy company culture, etc.
There will still be times that you simply can’t retain a great employee. For example, you just can’t match a big salary bump they’ll get with a new offer. Or, the mission of the new company is more meaningful for them than your own.
But, if you’ve been doing all the right things and someone is still ready to leave, then help them transition and support them. The industry is small and relationships span companies. We all end up working with each other again at some point!