Albert Barnes Commentary Acts 15:8

"And God, who knoweth the heart, bare them witness, giving them the Holy Spirit, even as he did unto us;" — Acts 15:8 (ASV)

And God, who knows the hearts (Acts 1:24). God thus knew whether they were true converts or not, and gave a demonstration that he acknowledged them as his.

Giving them the Holy Ghost, etc. (Acts 10:45–46).