Appropriateness

In the book "Give Praise to God" on page 276 Terry Johnson addresses the issue on singing psalms with appropriate tunes. He explains that the Scriptures ask us to use judgment on what is proper not just in tunes, but also in regards to the length of our hair, how we talk, and what we wear. My question is how do we know what is appropriate? Not only for tunes for the psalms, but our decisions in life. I know it is applying wisdom that helps us to make these decisions,but if you were to ask different people what they thought was appropriate tunes to put along with the psalms or even where they should draw the lines to what they consider modesty there would be a huge difference to what they thought was appropriate.