How Is Prophecy About the Messiah Recognized?

Questions about how to recognize prophecies about the Messiah in the Old Testament and whether or not Paul is just making Scripture say what he wants it to say when he appears to use Old Testament passages out of context.

 

• I heard a pastor connect Psalm 69:21 with John 19:28–30, saying it was Old Testament prophecy being fulfilled, but it doesn’t look like prophecy to me. How is prophecy about the Messiah recognized?
• How do we know Paul is not just making Scripture say what he wants it to say when he appears to use Old Testament passages out of context?