Saint Gregory of Sinai. "On Prayer: Seven Texts. On Delusion and Other Subjects". From Philokalia, Vol.4.
Question: What should we do when the devil transforms himself into an angel of light (cf. 2 Cor. 11:14) and tries to seduce us?
Answer: You need great discrimination in order to distinguish between good and evil. So do not readily or lightly put your trust in appearances, but weigh things well, and after testing everything carefully cleave to what is good and reject what is evil (cf. 1 Thess. 5:21-2). You must test and discriminate before you give credence to anything.
You must also be aware that the effects of grace are self-evident, and that even if the devil does transform himself he cannot produce these effects: he cannot induce you to be gentle, or forbearing, or humble, or joyful, or serene, or stable in your thoughts; he cannot make you hate what is worldly, or cut off sensual indulgence and the working of the passions, as grace does. He produces vanity, haughtiness, cowardice and every kind of evil. Thus you can tell from its effects whether the light shining in your soul is from God or from Satan. The lettuce is similar in appearance to the endive, and vinegar, to wine; but when you taste them the palate discerns and recognizes the differences between each. In the same way the soul, if it possesses the power of discrimination, can distinguish with its noetic sense between the gifts of the Holy Spirit and the illusions of Satan.