Some signs are obvious.
When I was less than two years in the pastorate, one of my key leaders met with me and said “I’m leaving the church.” The person left on good terms, did it the right way, and was released with a blessing. But, I took it hard. It is a part of the journey of growth as a leader. People come and go, and it is rarely personal.
It was clear to me, for some time, that this person felt they had outgrown my influence in their life. They were committed and faithful, but there were other intangibles communicating "my heart is not really with you". This did not make the person bad, it just was what it was. I ignored those impressions and did not have the emotional intelligence as a leader to manage it without deep prayer. Time and time again, my heart would be touched by similar people, until I learned to not ignore the obvious.
If you love deeply, people will provide opportunities for you to go through heartache. But if you don't ignore the obvious, you will learn to expect less from people and more from God. You will not expect from people more than what God allows you to discern about them. You will trust God as the Source of all the validation you need.