According to His will
When it's from God, you will know.
In the past, I use to struggle with this concept of truly believing that we will just know what's from God. But with learning to listen to God's voice, I was able to see and hear the way that God moves about in my life.
If we never take the opportunity to get to know God, we won't ever be able to hear Him. We will struggle through life, fall off the path that He has set for us, and have a difficult time with finding our purpose.
When we work on building a relationship with God, we can know His voice. It's like working on any friendship, over time you know how that person thinks, how they feel, and when they speak to you, you know it is them. In the same way, having a personal relationship with God, allows us to hear Him. We can hear Him in the whisperings of our heart when He tells us that He loves us. We can feel Him giving us strength in times of trial and weakness. We know when it is Him guiding our steps.
Building a relationship with God is the key to living a life of purpose. When we know God, we will be able to truly live a life according to His will.