“For to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace.” (Rom 8:6 ESV)
Basically, it would be so easy to discern whether we have set our mind on the Spirit or on the flesh if we only took a short glance into our hearts and check them for their peaceful or restless condition. The problem is, as soon as we sense that there is NO peace at all, we might be inclined to look at our own failures or to those of others instead of taking a short glance to God who allowed Satan to bring us into that confused, disheartened, sometimes even hopeless condition. We can be sure that the devil always tries everything in order to keep us restless and discouraged. He does not cease telling us about our wrongdoings or about those of others in order to keep our thoughts running like a hamster in his wheel – from past events to the future, back to the past, and so on and so forth. The sorrow and those negative feelings that go with these thoughts ALWAYS come from the devil, too.