This question can be broken into two parts. Let's start with the easier one. There are things which the Bible specifically declares as sins. Take a moment to read through the following passages: Proverbs 6:16-19, Galatians 5:19-21, and 1 Corinthians 6:9-10. Done? No, really. Go read them. I'll wait.
Okay, these verses very clearly list actions which God views as sin. There can be no doubt as to God's view on, say, murder, lying, stealing, etc. But where it gets more difficult is where the Bible isn't black and white, as in the above verses. While the Scriptures may not have a "thou shalt not" next to everything that might make you wonder if it is a sin, there are definitely principles available to help measure our choices against.
First, flip the question around. Instead of asking if something is a sin, ask this: Is it good? and Is it profitable? (1 Corinthians 10:23-24) If you're fumbling to find a reason to justify why you want to do what you want to do, then perhaps it is better left undone. Our days here are few, and our time is precious—too precious to be wasted on desires that have no purpose.
You may see this second point coming. Yes, it's obvious, but still very important: Pray about it. Ask God if He can use this activity or desire for His glory (1 Corinthians 10:31). Again, if you are finding yourself questioning how this thing could be profitable to yourself, much less how it could bring glory to God, perhaps it is best to give it up.
As believers, we have been bought with a price, we are no longer our own; our bodies are the temple of the Holy Spirit (1 Corinthians 6:19-20). We should strive to bring honor to God with how we behave with our bodies and our choices.
Next, ask this, "How will my actions affect my family or friends?" Is the particular thing you are wrestling with harmful to you personally? Maybe you're not convicted that it's sinful, and you truly mean to do it for the glory of God. But will it cause someone else to stumble in their walk with Him? God wants us to do our best to make sure our actions do not cause other believers to stumble (Romans 14:21; 15:1).
That may be your concluding thought after reading this article, and I am very sorry if you have that impression! But if you take nothing else away, please remember this major point:
Nothing—no habit, no recreation, no goals—should be allowed to have control over your life.
We have given that control to Christ. First Corinthians 6:12 says, "All things are lawful for me, but not all things are profitable. All things are lawful for me, but I will not be mastered by anything." Your walk with Christ is not an act of following a rule book; it is striving to more like Jesus and helping your fellow believers along the way.
Put your questions of "is this a sin?" through the filter of "would this be pleasing to Christ?" instead of trying to see where all the fences have been put up. Don't focus on the limitations of what you can and cannot get away with, but rather see every decision as an opportunity to live your life in a way that Jesus would.
Nothing should be allowed to have control over your life—except for God Himself. You may have a lot of freedoms in Christ, but that doesn't mean that you can do whatever you want (1 Corinthians 6:12). Instead of asking, "Is this a sin?" ask yourself, "Would this be pleasing to Jesus?" Don't focus on the dos and do-nots, but rather see every decision as an opportunity to live your life in a way that Jesus would.
Heidi Joelle spends her days staring at paperwork and making sure it is where it is supposed to be, how it is supposed to be, when it is supposed to be. And then she comes home and makes sure the porky little dog isn't eating a trashcan. Between these two events she tries to learn and see as much of the world around her as possible.