A.T. Robertson 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)

Which knoweth the heart (καρδιογνωστης). Late word from καρδια (heart) and γνωστης (known, γινωσκω). In the N.T. only here and 1:24 which see.

Giving them the Holy Spirit (δους το πνευμα το αγιον). And before their baptism. This was the Lord's doing. They had accepted (11:18) this witness of God then and it was true now of these other Gentile converts.