Discernment, for the Christian, has several different names in use. Many times these names overlap or replace one another so they need to be defined. Some of the terms I've seen in use are: Wisdom, Knowledge, Spiritual, Biblical, Worldly, Rational, Christian, Learning, Judgment, Instruction, Understanding, Prudence. For my purposes I'm going to consolidate them into three types: Spiritual Discernment, Rational Christian Discernment, Rational Human Discernment. Some could consider the second one to be a hybrid or subtype of the other two where creation and Creator mix, but let's go with these definitions and see what we get.
Rational Human Discernment (RHD): This is the only one we don't find in a positive sense in the Bible. Many are the verses that speak of the foolishness of the wise when compared to the things of God. You will find some who profess Christianity and misuse these verses to rail against any education throughout history and even up to today. They are wrong as we will see below.
What is Rational Human Discernment? Simply put it is that understanding given to all men from God. See Romans 1:18-25. God has given every man that much discernment, to be condemned for willfully failing to follow God. Some will give the excuse that they don't see enough, but at the correct time they will stand convicted in the Presence. Is there any use to RHD? Yes, because as stated in the passage above, it leads directly to God. In fact, anyone who has spent time studying philosophy should marvel at the glories of human reason. It isn't always so, but the best of it is so beautiful and sublime that it looks like it could almost touch God. Almost. I'll say that again, Almost. When viewed through the eyes of the ones who call themselves wise in worldly wisdom, they think they have achieved all truth, but from the eyes of the Christian, that 'almost' is a gulf that cannot be bridged and the greatest that can be done will still be nothing before the Almighty. It should still give some insight into the motivations of those not of the faith, because until they come under conviction, they cannot turn.
Rational Christian Discernment (RCD): Used in day to day living as the Holy Spirit leads us. Most of the mentions in the New Testament about discerning, regardless of the word used, are about this kind of discernment.
What is Rational Christian Discernment? It is the type of discerning that all Christians are commanded to learn in many places (Matthew 7:12-23, Acts 17:11, 1 Corinthians 5:9-13, 14:29, 1 John 4:1) and spoken about in Hebrews 5:11-14. It is a large portion of the maturing process all Christians are to go through as they are made perfect (mature) in Christ so that they are no longer tossed about (Ephesians 4:13-14) and stay true to the Gospel. Learning to use and exercise it brings forth the fruits of the Spirit as given in Galatians 5:22-23, Ephesians 5:9 and 2 Peter 1:5-8. It gives not only the ability to tell truth from lie, but to decide between what is trivial, important and essential. Simple example, the number of angels who can sit on head of a pin is a bit on the trivial side. Other examples abound, but a usual signal that the point is trivial is when people try to use their discernment to decide something that God has said is His to judge. The issue of salvation is a basic one and an excellent example for misuse since we cannot decide who is written in the Book of Life, only He can. He may let you know about you, but all you can do is declare it, not prove it objectively. Beware of misuse of this, since as soon as anyone says “How could any true Christian...” they have fallen into error as they are actually usurping God's authority and setting themselves up in the place of God to judge the hearts of others when they cannot even know their own (Jeremiah 17:9-10).
Spiritual Discernment (SD): It is special knowledge given to you by the Holy Spirit that could not be learned by human means. It will not contradict Scripture or reason, at least in the form of RCD, it may contradict reason in the form of RHD. See Matthew 16:15-17 for the clearest example, but also 1 Corinthians 12:4-11 and Ephesians 4:11-13.
What isn't Spiritual Discernment? Yes, this is a slightly different question than the other two, but it is the most misused claim by those that profess to be Christians. While the Spirit does direct Christians and He does so more and more as they mature, there is no Spirit “bell” in anyone that tells them everything, which is why we are to test every spirit (1 John 4:1) and teacher for false doctrine (Romans 16:17-18, 2 John 9). Of course, such doctrine isn't downloaded into all believers by magic and revealed to them by their feelings (I like calling it the twinge in the “Holy Ghost elbow”.), it comes from the study of Scripture. Although it is possible to know something is True by feeling it is, it will not be something that contradicts Scripture since God is not the author of confusion and those that claim it the most are those that come up with something that contradicts RHD, but also RCD. Often enough, they claim that all reason and logic is against faith (IOW, they buy into the world's view), they make claims about others' salvation, and refuse correction when they are caught in error and simply carry on while acting like they never got busted. Yes, this sounds like childish behavior, but we are talking about a maturity issue and for some reason this is the most claimed gift. As soon as you hear “I just know”, citing the testimony of others without Scriptural basis (or a contrived and shaky one) or claims of special knowledge not found in Scripture, be very, very careful because this is almost assuredly not Christian.
First, why do we want discernment/judgment/wisdom/understanding? Because not only do we have to discern between true and false, but to decide relevant and irrelevant (see Romans 14). Some games are simply not worth the candle and Satan must love Christians getting caught up in side issues while witnessing or on the other side, get so caught up in “pure” preaching of the Word that we forget the fact that part of our walk includes taking care of others (James 2:15-16). Of course, as noted in Scripture, we are commanded to seek it (Proverbs 4:5, 7; 16:16).
Second, how can we tell how we have a Christian level of discerning? Not necessarily simple, but we can do it by being fruit inspectors (Luke 6:43-44) for others and the Spirit will let you know when you have it, or at least all you can handle at this time remembering always that all spirits must be tested and that Scripture cannot be contradicted.
For me, it is simple as I have learned as the years go by, but hard to explain. The key revolves around the insights I get from Scripture, the peace that passes all understanding and seeing the Hand of God in all the ways of my life. It may include (and frequently does) turbulence on the worldly level, but the surety and Scriptural confirmation shows it to continue to grow. Am I finished and perfect (fully mature) in Christ yet? Given my past, I think not, but as I need more, it will be given to me, whether I take it peacefully or have it hammered into me.