We all should know and understand that. Show me your faith without deeds, and I will show you my faith by what I do. This is the test of faith, or for that matter, any virtue. Not even love can be shown without acts. All of us know those who are all talk and no produce.
Talk is cheap and action is costly.
There are any number of practical ways in which we can measure our faith. We can know how great our faith is by how much we sacrifice in giving to the Lord's work. We can know something about our faith's strength by how much time we spend with Him in His word and how much time we spend with Him alone in prayer. All of these things are a direct result of faith. We believe Him when He has commanded these and other things for our good and for His praise and joy.