Those filled with the Holy Ghost spoke in tongues. (Acts 2:1-4, 8:12-17, 10:44-48, 19:1-6)
Acts 2:4
“And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.”
After the above incident, Peter specified that what bystanders were seeing and hearing WAS the Holy Ghost that Jesus had shed forth as promised:
"Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear." Acts 2:33
Acts 10:44-46
“ While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
For they heard them speak with tongues,”
How did they know the Gentiles had received the gift of the Holy Ghost? The answer is recorded in verse 46, “For they heard them speak with tongues.”
Acts 19:6
“And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues…”
Acts 8:12-18
"But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women.
13 Then Simon himself believed also: and when he was baptized, he continued with Philip, and wondered, beholding the miracles and signs which were done.
14 Now when the apostles which were at Jerusalem heard that Samaria had received the word of God, they sent unto them Peter and John:
15 Who, when they were come down, prayed for them, that they might receive the Holy Ghost:
16 (For as yet he was fallen upon none of them: only they were baptized in the name of the Lord Jesus.)
17 Then laid they their hands on them, and they received the Holy Ghost.
18 And when Simon saw that through laying on of the apostles' hands the Holy Ghost was given
Notice Acts 8:12-The Samaritans believed Philip’s message. But they did not receive the Holy Ghost at that time. How did the people know that they had not received the Holy Ghost? See verse 14, Peter and John were sent for in order for the Samaritans to receive the Holy Ghost. Also, ask yourself. What did Simon SEE that convinced him that the Samaritans had received the Holy Ghost? (Verse 18)