We can know God's will for healing because His will IS His Word!
1 Peter 2:24 (NKJV) says "who Himself bore our sins in His on body on the tree, that we, having died to sins, might live for righteousness--by whose stripes you were healed."
We can declare that we've already been healed 2,000 years ago by the stripes of Jesus because God's word says so.
If we put the doctors report or any other health advice above God's Word, we will only receive the thoughts and principles of this world.
Does it make sense to listen to the world's advice and yet expect healing in our body?
If you want to walk in divine health, you'll need to learn what God says about healing in His Word.