This means saying yes to God. Nothing less. Please don't even entertain another option. Your question might be how God speaks definitively, but when He does, you must serve. I have heard people say, "when He calls me on the phone, I'll know for certain what to do." I see this as a humorous attempt to dodge the realities of faith.
For those who lean towards that position, they wouldn't know what to do if He were on the phone. You see, He speaks to us in a variety of ways. It may be a voice in our heart or a compelling circumstance. It may be a child through whom He speaks or a stranger. Look for Him. Listen to Him...and do what He bids. For God's sake do it! (and for yours)
Deuteronomy 13:18 (MSG)
18 Yes. Obediently listen to God, your God. Keep all his commands that I am giving you today. Do the right thing in the eyes of God, your God.