How did Peter and the six Jews with him know the Gentiles had received the Holy Spirit?
They knew because they heard the Gentiles speaking with known languages and magnifying God. The six Jewish believers accompanying Peter witnessed the Holy Spirit poured out on the Gentile household of Cornelius, evidenced by their supernatural speech in recognizable languages rather than an unknown tongue or guttural sound.
Les explains that as many as came with Peter, these six Jews, saw that on the Gentiles also was poured out the gift of the Holy Spirit. How did they know? For they heard them, the house of Cornelius, speaking with known languages, and they magnified God. Les points out that they were not talking to God but to fellow men, magnifying God in the presence of these seven Jews and the members of Cornelius's household. There was no manifestation of an unknown language or a guttural sound; it was speaking languages. This was the second time the Holy Spirit was manifested by giving the gift of speaking more language than what they normally lived with. The first time was to Jews in Acts 2, the second to a whole house of Gentiles, and the third would be another unique little group in Acts 19.
More Questions
How did the Gentiles in Acts 10 show that the Holy Spirit had come upon them?
The Gentiles at Cornelius's house had believed, and the Holy Spirit came upon them evidenced with tongues, which was a manifestation that the Holy Spirit had come upon them.
Why was Peter so shocked when the Holy Spirit fell on the Gentiles?
Peter was flabbergasted because the order was wrong: he expected repentance, then baptism, and then the Holy Spirit to come, but none of that happened—the Spirit came while he was still preaching.
At what point did the Holy Spirit come on the Gentiles in Acts 10?
In Acts 10, the order was totally reversed for the Gentiles: the Holy Spirit was already being evidenced before Peter had finished preaching, before they were baptized, and before he heard their repentance.