1 Corinthians 2:12 Now we have received not the spirit of the world, but the Spirit that is from God, so that we may understand the gifts bestowed on us by God.
Our comfort with physical science allows us to think that real things must be measurable. We can measure distances. We can observe the effects of forces exerted between objects due to gravity, electricity, magnetism, and even nuclear forces. Magnetic Resonance Imaging (MRI) used by doctors daily relies on measuring the change in the spin of subatomic particles, a phenomenon that cannot be directly observed.
What cannot be measured is the interconnections between each person and God, all spiritual gifts depend on this one gift.
How do I know this gift is real? The same way I know that the images produced by MRI are correct. They are consistent. The gifts of the spirit consistently produce fruit for the building of the Kingdom of God.
Where have you seen God immeasurably at work?