This is a very good question that every person should ask and find an answer to, especially if we are concerned about living a life that is pleasing to God. We will approach the answer from several different directions.
Firstly, There are Many Things That the Bible Defines as Sin
For examples check out Galatians 5:19-21, & Exodus 20:3-17 (no seriously, check it out, I’ll wait) and many others. Rather than attempt a rule-book approach to a righteous walk, we should reverse the question and ask if the act we are considering is good? Does the act/attitude in question do harm to others? How will the act impact my family and friends? Will it impede my relationship with the Lord? In this way we challenge not only the sinful activities in question but also challenge the unfruitful actions we may be involved in.
Secondly, We Must Ask How Does the Act Impact Your Conscience?
A general rule of thumb is that if it is an activity that you would hide from your Pastor or your parents, it is something that you know deep within is wrong and is therefore sin. We see this throughout human history. When Adam sinned he hid himself from God. When King David sinned with Bathsheba he attempted to hide it even to the extent of murdering her husband. But to dig deeper, if you are even questioning whether or not an activity is sin reveals that apprehensions already exists as to the godliness of the activity. To have a crystal sharp discernment of what sin is also requires a humbling honesty concerning one’s own brokenness.
Thirdly, How Does the Action/Attitude in Question Impact Our Loved Ones or Those Who Love Us?
We can often see a reflection of the degree of godliness in our action or attitude by looking at how those who love us feel about our engagement in that action or attitude. Some sins, especially those of attitude, can have a devastating effect on our families and friends.