After they were trained they were tested. The tellers were put in front of a conveyor belt loaded with genuine bills. Occasionally someone would insert a counterfeit bill onto the belt. The tellers would immediately look at it, pull it off, and say, “I can't tell you why, but something's not right about this bill. This must be a counterfeit."
The reason they were able to detect the fakes was that they had spent so much time with the real deal. They knew what the genuine article was all about so anything that didn't match up to it exactly stuck out like a sore thumb.
The same principle holds true in our lives. We are bombarded with so much information today that it is sometimes difficult to discern what is true and what is not. The Word of God is truth. Just like these tellers, we need to spend time in it so that we know what is genuine and what is fake.
If you want to see a cool way that I recently learned to do this, here is a link to a message I gave in our church a couple months ago.
I have hidden your word in my heart that I might not sin against you.