“How do we ascertain if they are NOT ready for it?”
Plant seeds. Speak in parables...
But for the most important knowledge, guide them to go within in prayer and meditation to get it direct through the Holy Spirit.
A person must know enough to ask the right questions before you can answer with the highest knowledge.
For example, a person who is given the ability to heal is also given the ability to harm. Just as Jesus healed, but also cursed the fig tree and it lost its leaves.
Jesus spoke in parables. Those days are past.
We are to preach the gospel as He commanded so that souls can come to Him. He is not hiding Himself from mankind in this church age.