How can anyone really know? Most people will likely say they knew When they got married to their wife or husband but with the divorce rates the way they are that’s not a good answer for me.
This is a good thing! One should certainly not get married without strong assurance they are making the right decision. And once they do, they should not consider “divorce” an option.
Also there are some priests who left just so they can get married so I guess they didn’t know either.
There is a very long period of discernment and a candidate takes temporary vows. Everyone has to agree, the spiritual director, confessor, vocation director and if it is a community, the members. Priests that leave were not necessarily uncommitted to their vows. I know some priests who have left after decades of faithful service. In some cases they drift away from supports they need to keep their vows.
So how can you know for sure
When you go through the discernment process, clarity will come, one way or the other. In any case, as long as you are following God, remain in a state of grace, and are obedient to those who are trying to help you discern, you can follow your conscience with peace.
For some people, the process itself is God’ intention, not the result. People sometimes stay in formation or live with a community for some time before they decide not to stay.
I wasn’t raised Catholic; but, I knew I wanted to have children so religious life wouldn’t have been a consideration even if I had been.
This is a good point. Those who are called to marriage/family usually have a desire to be married and have interest in children. This is not to say that persons called to vocation never do, but a desire to have children that starts very young is something to consider, just as much as a lack of desire for these things. One of the reasons I thought I was called to religious life was a complete lack of interest in dating, marriage, kids, etc.