He who follows the spiritual path must pay great attention to discrimination, since the ability to distinguish between good and evil, and to scrutinize and understand the various tricks through which the devil by means of plausible fantasies leads most people astray, keeps us safe and helps us in every way….
For the devil cannot bring about love either for God or for one’s neighbor, or gentleness, or humility, or joy, or peace, or equilibrium in one’s thoughts, or hatred of the world, or spiritual repose, or desire for celestial things; nor can he quell the passions and sensual pleasure. These things are clearly the workings of grace. For the fruits of the Spirit are love, joy, peace and so on (Galatians 5:22), while the devil is most apt and powerful in promoting vanity and haughtiness.
You know from its effect whether the intellectual light shining in your soul is from God or from Satan.
~St Makarios of Egypt (paraphrased by St Symeon Metaphrastis)