There are many times that there is no clear answer. If you ask a friend to pray for you and then the friend provides the thing you want, there doesn't appear to be proof that it is also an act of God. People who believe in God won't take much convincing to believe that God used your friend to provide for your needs. Someone who is skeptical of prayer will say that the friend heard of the need and wanted to help . No matter which one you side with, it all hinges on what you believe.
Hide in the Closet
If you want to know that it is God who has answered your prayer, don't tell anyone else what you are praying for. Matthew 6:6 says that when you pray you should go into your closet and shut the door. "And they Father which seeth in secret shall reward you openly." Though there is nothing wrong with people asking other people to pray for them, but if we want to be sure that it is God who has answered our prayer and not someone else, he should be the only person we tell. Then, when a friend who has no idea that we've been praying for a thing comes to us and gives us the thing for which we've been asking, there is no doubt but that God heard and answered our prayer through the friend.Ask For Something Only God Can Give
When you use a credit card or write a check at the store, you are required to sign your name. By doing so, you are telling your bank that you are the one requesting that they give the store the given amount, rather than someone else. The reason you are asked to sign your name is because you sign your name differently than anyone else. It is difficult for someone to match it, but it is a very simple thing for you. If you want to verify that your prayer has been answered by God, ask him for something that only he can provide.Be Specific
It is tempting to pray a generic pray that is rather vague about what we need. Then we might end it by telling God "not my will but yours." That may sound spiritual because that's what Jesus said, but it does nothing for us when we're trying to see God's power to answer prayer. Jesus had a very different situation than we do. Being God, he could have at any time stopped on his journey to the cross and said, "that's it, I'm done, I'm not going any farther." He qualified his request in the garden by saying, "If it be possible." If his prayer had been simply "take this cup from me" it would have been over, but he completed the journey. We aren't in that same situation. We aren't God. If we ask for something outside his will, he isn't obligated to give it to us. But when we're specific in our prayers and God gives us exactly what we've asked for, it gives us evidence that he hears our prayers and he blesses us.That's not to say that we should say something like, "Lord, show me that you answer prayer by giving me $1,234,567.89 tomorrow at 10:11AM." It isn't that God isn't able to do exactly that, but we shouldn't try to manipulate God. Rather, when we find ourselves in a position that we're praying for something anyway, let's be specific in our request so that we can see that it is God who answered our prayer. It is still up to God whether to answer our prayer or not and in the way we ask or not, but when he answers it in the specific way we ask we are able to see that he is strong on our behalf.