That is an excellent question, Opal. How I would like to hear more people ask that question, before they jump out into action - too quickly, even “prematurely”.
The disciples knew, when the Holy Spirit told them. He, the “power from on high”, speaks an unmistakable command - and a disciple knows that God has spoken to him.
There is a brief book of traditional Catholic spirituality that can be found still - It was originally printed as The Three Ways of the Spiritual Life, by Fr. R. Garrigou-Lagrange.
Amazon has it as:
The Three Conversions…
but you can find a free download if you do a search.
It will explain the process of maturation in Christ, very well. It can change your life, to learn this, from the saints of our Church, which is mostly not recognized among many, many “teachers” in the Church today, I’m very sorry to say.