It's fairly easy to determine when I'm thirsty and it's normally an easy fix. Grab a glass, fill it up with milk or water or lemonade (or margarita), and drink. Viola! Thirst subsides.
How do you know when your soul is thirsty? What are the symptoms? What is the cure?
Perhaps it's best to begin with why the soul thirsts. The most obvious reason we thirst for God is because of sin. Mortal sin, especially, places a lock on the spiritual fridge where the ice-cold water for the soul is stored. All sin separates us from God, but mortal sin severs our ties with God. Think of it way: a venial sin makes us like a 2 year old who can open the fridge door, but we're not tall enough to reach the shelves. Mortal sin means we've been banished from the kitchen entirely because we reached up and grabbed a knife off the countertop; we think we don't need to listen to Daddy's rules, therefore we are kicked out so that we won't do further damage.
The only way I can get access to God's Kitchen after I have sinned mortally is to seek the Sacrament of Reconciliation, confess my sins with genuine remorse, and strive to never sin again. If I confess a mortal sin thinking, "Yeah, I know I'm gonna' go do this again next week", then I'm really not looking to change my life and my relationship with God is not going to be restored.
The second reason we experience spiritual thirst is because it's part of our DNA. God made me so that I would thirst for Him. As St. Augustine said, "Our souls are restless until they rest in you." God always wants a deeper relationship with us so sometimes He will withdrawn from me - just a bit - so that I will go deeper into my prayer, take my relationship more seriously.
Symptoms of a thirsty soul: The restlessness that St. Augustine refers to, asking questions about the faith, wanting to "do" something to be closer to God, thinking about going to Confession. These are just a few of the signs.
The cure: prayer. One word answer, but not an easy answer. A good way to start is with the Lord's Prayer - and really pray it, think about the words Jesus gave us. Why did he want us to pray in this way?