We can tell whether we have the right kind of faith. Hebrews 11:1 provides a definition: "Now faith is the substance of things hoped for, the evidence of things not seen." Hupostasis, the word translated "substance," means "that which underlies the apparent; that which is the basis of something, hence, assurance, guarantee and confidence" (Spiros Zodhiates, The Complete Word Study Dictionary: New Testament, p. 1426). The English "substance" is built from a prefix and a root which together mean "that which stands under." Webster's defines it as "the real or essential part or element of anything; essence, reality, or basic matter." It is very similar in meaning to hupostasis.
Paul is saying that, for Christians, faith underlies what is seen externally in the conduct of their lives. Underlying a building is its foundation, and in most buildings, the foundation is rarely seen. If it is seen at all, usually only a small portion is visible, but it is there. If no foundation exists, the building soon becomes crooked and warped. In most cases, it will collapse and be completely unusable.
Since Paul says, "We walk by faith, not by sight," we understand that underlying the conduct of a Christian's life is not merely believing that God is, but a constant and abiding trust in Him. Since it is impossible for God to lie, we trust that what God has recorded for us to live by is absolute and must be obeyed, and that it will work in our lives regardless of what may be apparent to the senses.
How much of what we do is really motivated by an implicit trust in God's Word? This is how we can tell whether we are living by faith. We must be honest in our evaluation though. We find it very easy to shade the truth through self-deception. We justify disobedience by rationalizing around God's clear commands or examples, saying that our circumstance is special because . . . (fill in the blank).