Q: How do we know when decisions in life are ones we need to hear from God about before acting or if they are something we just need to take care of ourselves?
Your question is pretty common, and yet I think there's a fundamental misunderstanding about this sort of thing in the minds of most Christians.
Walking with the Lord is not so much a matter of figuring out when and where you need to ask for guidance. It's really more about having a surrendered heart that desires the Lord's leading even when you don't ask. As the Lord spoke about Caleb in Exodus 14 He said, "But my servant Caleb...has a different spirit and has followed me fully..." Numbers 14:24 (ESV) The Lord called attention to Caleb's heart attitude to simply follow the Lord in all things.
Like Caleb, we ought to be desiring God's will and direction every single day — and our prayers should reflect our desire. "Lord, lead me today and guide my life as You think best. I give my day to You and ask that You would open and close doors according to Your perfect wisdom." Then, as we go through our day, we can walk in the expectation that He is leading and directing us. (See Psalm 32:8)
Now obviously there are some really huge, life-changing decisions that we encounter from time to time, and some of them may require us to wait for a very specific directive from the Lord before preceding. And I think we identify those kinds of decisions by quieting our hearts in His presence and spending time in the His Word.
But even if other decisions seem small and insignificant, I would never want to think of myself as taking care of business on my own. I believe the Lord always wants His children to walk in a surrendered attitude toward all our decision-making, and to pray for His guidance each day. Then we can step out into our day with an expectation and confidence in the Lord's leading.