The way we typically measure temperature is not really a direct measure of heat. Your typical mercury or alcohol thermometer really measures the amount of space displaced by the liquid in the vacuum sealed cylinder. Someone figured out that liquids expand when place under heat. You can see this every time you boil water in a pot over a stove range. The hotter the flame the higher the water will raise in the pot. The same amount of liquid now occupies more space.
That is how a thermometer works. The hotter it is the liquid inside the cylinder expands, the colder it gets the liquid contracts. This phenomenon is predictable and reproducible. This is why using these principles they can build thermometers.
Faith is also measured indirectly. Christ told his disciples that a little faith can accomplish great things, we read in Matthew 17:20, "And Jesus said… If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you." Christ established that our faith will be evident to others. James famous discourse on faith and works is often quoted on this subject,
James 2:14 What doth it profit, my brethren, though a man say he hath faith, and have not works? can faith save him?
James 2:17 Even so faith, if it hath not works, is dead, being alone.
James 2:18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.
So many say, works is the proof of faith. Paul says we are not saved by works. In other instances Christ seems to agree. In Matthew 7, Christ tells the disciples,
Matthew 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Matthew 7:22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Matthew 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
The works of the people mentioned here did not show faith. However, Christ still insists, "Wherefore by their fruits ye shall know them" (Matthew 7:20); a concept that seems reiterated in John 15 in connection with a display of love or agape. Christ tells the disciples in John 13,
John 13:35 By this shall all men know that ye are my disciples, if ye have love one to another.
So, it seems to be that the way in which we measure someone's faith is by His love in us toward others. In 1 John we find a connection between faith and love. By studying the passage from 1 John 4:12 through 1 John 5: 5 we can arrive at this statement: "By faith we overcome when we are born of God, Who is love and we cannot see, but He dwells in us (and us in Him) and casts away our fear." So, we can conclude that where there is faith there is no fear for love casts it away. So, in the presence of self-less, self denying, unconditional, perfect Love there is no fear and only faith. How do we know when this love is in us? When 1 John 3: 16 is true about us: "Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren." It takes faith to do this.