Test and verify, and then repeat the test. If it is true, you should get the same result – over and over again.
Prime example – one foot is one foot. You want to run a 7.1 mile race/jog/walk, you have to go the distance. Racers running a marathon do not get to choose when they have finished the race – they have to cross the start and finish line. The last time I walked "Bay to Breakers" in San Francisco, each entrant had a little plastic tag that had to be attached to our shoe. When we crossed the start line, our "start" of the race was logged in a computer. When we crossed the finish line, our "finish" was recorded and our race time computed.
Imagine a racer who never crossed the starting line – what would be their "finish" time? A little hard to imagine, but those were the rules for the race. There were even "Course Monitors" who's job was to make sure racers did not enter from the sidelines or take shortcuts. To finish, you had (or were supposed to) run the whole race.
There are rules to everything – football, basketball, soccer, you name it. But let anyone say there are rules to life and you get arguments. Well, reading First John, Chapter 4 leaves no questions that there are rules to "following Jesus."
He talks about following the truth – the rules for living the Christian life. Knowing those who are and those who are not is a matter of knowing the rules. You are going to say "It's not my place to judge…" That's all well and good, but John says here "test the spirits", and Paul says "examine everything carefully…abstain from every form of evil. (I Thess 5:22)" And to top it off, Jesus said "You will know them by their fruits. (Matt 7:16)"
Okay, I will give it to you that they were certainly more "holy" than me, so they could more easily know the rules and make those "judgment calls". Maybe the answer is that we need to be more "holy", then we could test the false teachers. The problem there is "how do I become more holy?"
Then, maybe that's not how the test is done. Maybe it's just having the rule book, reading it and applying it to every area of life. Call a foul a foul, a strike a strike and a touchdown a touchdown – by the book. If you have the book, you can determine if someone is telling the whole truth.
The question now, is am I reading the rule book or just carrying it in my pocket?