Faith to pray for healing?

Have you prayed earnestly for someone who is sick? Even praying for recovery with an earnest belief that God can certainly intervene and heal any illness? James 5:14-15 says: 

Is any one of you sick? He should call the elders of the church to pray over him and anoint him with oil in the name of the Lord. And the prayer offered in faith will restore the one who is sick. The Lord will raise him up. If he has sinned, he will be forgiven.

But what is faith?  Hebrews 11:1 says:

Now faith is the assurance of what we hope for and the certainty of what we do not see.

Do we have from God the supernatural assurance and certainty of a particular healing?  Enough to declare it publicly as accomplished, speaking as it were the very words of God Himself?  

We are told by Jesus in Luke 17:5-6 that the size of our faith doesn't matter, only the presence of it.  We are also told that one of the many supernatural gifts of the Spirit is a "word of faith" (1 Cor. 12:9).  

In other words, we can't "gin up" the faith by gritting our teeth, knitting our brow and straining for it - God either gives that supernatural faith or He doesn't. I think that the "prayer offered in faith that heals the sick" is one where God, miraculously, has revealed to me His explicit intent to heal a specific person at a specific time, and revealed it with such clarity that I can state it with total confidence, with no doubting at all. (James 1:6) 

This is not something I can do, no matter how great my desire for it - only God can grant that revelation of His intent, through His Spirit.  If God has not given me that clear and unwavering faith, however, I can still pray earnestly and trust that the Holy Spirit will curate and translate my weak and incoherent prayers according to the will of God, as promised in Romans 8:26-28:

In the same way, the Spirit helps us in our weakness. For we do not know how we ought to pray, but the Spirit Himself intercedes for us with groans too deep for words. And He who searches our hearts knows the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God. And we know that God works all things together for the good of those who love Him, who are called according to His purpose.

I can still pray with great confidence that God hears me (through my advocate, the Spirit) in the courts of Heaven, even if I lack clarity on exactly what and how to pray.  Groans are good enough!