How do you know if your prayers have been accepted?

If I pray sincerely, I do feel a calming feeling afterwards, but unfortunately I find it difficult to pray with 100% concentration all the time. But I get what you're saying, I feel like Allah is pleased with me on certain days when I feel energized, and I feel like praying sincerely, and reading Qur'an sincerely, and tacking each of my responsibilities with the help of Allah by remembering Him. There's no way for sure to know if a prayer has been accepted, but remember that we get a portion of the reward of Salah, depending on the sincerely and devotion we prayed it with.