John 14:10
ESV - 10 Do you not believe that I am in the Father and the Father is in me? The words that I say to you I do not speak on my own authority, but the Father who dwells in me does his works.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
1 John 5:15 tells us He hears when we ask according to His will. We are also told by Jesus, if you abide in Me and My Word abides in you, ask what you will, and He will do it for you, that it brings glory to the Father. Oh and he also mentioned in that verse ( I think in John 15) that we must bare good fruit, fruit that will last too....and we also must remember we are told if our hearts do not condemn us, (because if we have un-repented sin in our lives our prayers can be hindered).
The best way to know your prayers are heard is by first asking Jesus to be Lord of your life, then living a submitted life to Him, and asking Holy Spirit to help you to pray according to Gods will.