But how come Peter knew what Ananias promised in his heart?

It is possible that Peter had received divine revelation or insight from the Holy Spirit, allowing him to know what Ananias had promised in his heart. Additionally, Peter may have been able to discern Ananias' intentions based on his actions and behavior. Ultimately, it is not explicitly stated in the biblical text how Peter knew what Ananias had promised in his heart, but it is likely a combination of spiritual discernment and observation.

Peter knew what Ananias promised in his heart because the Holy Spirit revealed it to him. In Acts 5:1-11, Peter accused Ananias of lying to the Holy Spirit and keeping back part of the money he had promised to give to the church. The Bible says that, "When Ananias heard these words, he fell down and died" (Acts 5:5). This shows that the Holy Spirit knew what Ananias had promised in his heart, and that He revealed it to Peter.