The Holy Spirit is invisible to the natural eye. The Holy Spirit was compared by Jesus to the wind:
The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit. (John 3:8)
Although the wind is invisible, the effects it produces can be seen and heard. When the wind blows the dust rises from the ground, the trees all bend in one direction, leaves rustle, the waves of the sea roar, and clouds move across the sky. These are all physical signs or evidence of the wind. So it is with the Holy Spirit. Even though He is invisible, the effects which the Holy Spirit produces can be seen and heard.
2. HOUSE OF CORNELIUS:
Other supernatural signs of the Holy Spirit are mentioned, but none of these were evident on all three occasions.
From these Biblical records it can be concluded that the physical sign of speaking in tongues through the power of the Holy Spirit confirms that a person has been baptized in the Holy Spirit.
For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries. (I Corinthians 14:2)
Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered. Romans 8:26