More often than you might think, people ask me whether or not I think their temptations and struggles are of diabolical origin.
Most of the time I don't think so, but I tend to take the practical approach: bracketing off the question of the devil's possible role, I ask whether or not guessing him to be involved will help someone take action. In other words, I ask myself whether the idea of the diabolical will tone someone up for fruitful spiritual combat or further absolve him from taking responsibility for himself.
That being said, there are a couple of cases of spiritual erring and sin where I am inclined, at times, to consider diabolical influence:
1. When someone is "inspired" to be intensely religious, but in such a way that is not about God, but about himself. I think it's a mistake for a devout person to think that the devil will try to tempt her to sin in ways that are obviously crude and worldly. It is the devil's joy to have us be as religious as could be, so long as we do it his way. So the diabolical attacks on devout people take the form of temptations to spiritual vanity, self-righteousness, abuse of church power and control of others, etc. To form a very religious person who only reflects on the sins of others and the punishment they have earned for themselves, a proud religious, or a priest who delights in vanities, power, or ambition, are examples of the devil's proud accomplishments.
2. In certain temptations that make a tricky inversion of self-indulgence and humility. Say someone is working hard at the spiritual combat against a habitual sin, like an addiction of some sort. A suggestion comes into his mind saying, "Just sin and get it over with, then you can at least pray in repentance." Of course this is a trick because the good of repentant prayer--which is a familiar place for our victim--is set up (falsely) as a way to get him to give up the fight. His greater familiarity--and hence comfort--with the state of praying as a repentant sinner is used to get him to give up the newer and much more awkward and uncomfortable position of being in the spiritual combat.
UPDATE: Credit where credit is due. Since this morning I have begun to suspect that I got at least the idea of 2. from Fr. Benedict Groeschel's The Courage to be Chaste. I can't check or give a citation because I gave away my copy some years ago. Nevertheless, it's a good book and I recommend it.