Devils can inflame your passions and present temptations in your imagination. They can also interfere with your mental associations, i.e., they can make you associate certain things as pleasurable, and certain other things as unpleasurable. They can draw from your memories and bring the worst ones out to torment you or tempt you.
For example, they can inflame the passions to cause a very pleasurable association with another person and so an adulterous relationship or a relationship outside of wedlock. They can cause an unpleasant association with any thoughts of Catholic teaching, and pleasant associations with heresy and liberalism.
How often is it the devil and how often is it your own vices alone?
This takes mental awareness and introspection – paying attention to what you will, and what your habits are, and what arises spontaneously and without your intervention.
Most new Christians immediately become aware that gaining various virtuous habits is a matter of fighting for it, and what they thought would be easy, because before they were not interfered with – the Devil had them completely his own – but now, as they try to escape sin – it is unexpectedly difficult.
But the gift of spiritual discernment, that of knowing always when it is due to spiritual intervention, is one that is normally only gained through experience and constant work at attentiveness, and along with gains in knowledge about what is virtue and what is vice. Over time it becomes more developed.
You can help yourself in spiritual discernment through using St. Benedict medals, as well as saying devoutly -aloud-, the holy names – that is the names of the holy family. For example, Jesus, Mary, and Joseph, help us – said -devoutly- and -reverently- rather than misused which is a sin that is causing great evils today.
The good angels, like our guardian angel, on the other hand, work differently, guiding us to what is truly good. Contrition, peace, purity of intention and virtuous habits are quite important to being open to inspiration from Heaven.