Unfortunately for us, God does not take out billboards that blink in bright lights, "DO THIS!" (Not usually. Probably not. But don't discount it.)
What to do? Here are a few things I've learned about discernment:
1. First - and always - are you trying to choose between two good things? I know that sounds obvious, but sometimes we delude ourselves. Make sure you really are trying to decide between two goods.
2. Pray. A lot. In silence. God's voice gets terribly muffled in the din of our days. If you can, go before the Blessed Sacrament and spend time in prayer.
3. Don't push it. Unless it's an emergency, just keep taking your discernment to God in prayer and let Him guide you.
4. If you are able, talk it over with a spiritual advisor. If you don't have one, try a trusted priest or religious.
5. Ask others to pray for you. They don't necessarily need to know what you're trying to decide; they just need to support you in prayer.
6. Check and see if God has answered your prayer. Sometimes we think that God's voice is just going to "appear" in our thoughts and say, "Here's your answer." However, God often puts things in our daily path that are pointing us in the direction He wishes us to go. Be aware of those things. What's in your mailbox? What are people talking to you about? What are you hearing when you are in prayer or spiritual reading?
7. Ask the intercession of a saint. These people stand before Almighty God. You can't ask for a better prayer partner.
8. Don't push aside the hard answer. Often, we know what God wants, but we just don't want to do it - it's too hard. If you are truly willing to do what God wishes, you have to be truly willing to do the tough, the hard, the awful, the painful. Remember the rich young man? Jesus asked him to sell all that he owned and follow Him. The young man could not do it, and scripture tells us he walked away from Christ, sad. Don't do that.
Discernment is a process, not an event. Allow God to work in His time, which is not our time.