The kingdom of heaven is likened unto a man which sowed good seed in his field:
Mat 13:25 But while men slept, his enemy came and sowed tares among the wheat, and went his way.
Mat 13:26 But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
Not knowing what a tare was, I did some research. Tares are basically weeds that look like wheat only they bear nothing good. Wheat produces edible grains, which can be used to feed people. Tares, however, produce little black seeds, that if eaten cause paralysis, nausea, intoxication and depending on how much is eaten, convulsions and even death. They are poisonous. The root system of the tare is such that it attaches itself to the root system of the wheat, so carefully intertwining with the roots of the wheat it cannot be separated without pulling up the wheat. But it has to be carefully, deliberately planted to achieve that result. At first sight, you can't tell them apart. It's only when the wheat begins to mature and bear fruit, and the tares mature that their lack of fruit reveals what they are.
Mat 13:27 So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
Mat 13:28 He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
Mat 13:29 But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.
Mat 13:30 Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
Only the Lord can tell the difference. His Spirit gives discernment as a spiritual gift and in this day and age we desperately need to be asking Him for this gift. He lays out the test-it's by their fruit-by what is produced and evident in their life. The fruit of the Spirit of God is listed in Gal_5:22-23. If that is evident in someone's life it's because the Holy Ghost poured out the love of and for God in their hearts and it manifests itself in kindness, gentleness, long suffering, patient endurance, meekness, goodness, joy and peace. These things because they depend on the unchangeable Lord, are not conditional or dependent on circumstance. When they are pretending, and only look like Christians, and talk like Christians, yet you see no evidence of the Spirit of God in their life, that is a giveaway. Remember, the wheat produces grains because the Lord sends the sun and the rain in the right measure to make it grow. The tares receive the same nourishment but bear no fruit-because the two are different. They resemble each other, but one is good and one is bad. Their natures are different. Someone with a heart for God and someone whose only interested in self can sit under the same anointed message by a yielded, obedient servant of the Lord with very different results. One will be humble and broken, and hungry for more, yet the other unchanged and unmoved even though outwardly they may appear to be religious.