How can I tell good thoughts from bad ones?

Father Andrew: There are thoughts that come from God but they are not even thoughts, they are more like inspiration. There are simple thoughts about our daily lives, and they originate in our own hearts. In addition, there are evil devil-inspired thoughts. How to tell the difference between them? Perhaps, due to the fact that we are humans, we cannot avoid some mistakes but if you seriously, with all responsibility, with all your heart and reason, ask yourself whether this or that thought comes from God or not, you will immediately get the answer because somewhere in the depth of your conscience you will know it.
I think that we all know everything. It is just that we are too lazy to take the effort to discern thoughts. When an individual is filled with lots of thoughts, emotions, dreams and plans, it appears to him that it is impossible to differentiate between them.
If you do not give food to a certain person for a couple of days, his thoughts will be very limited in scope. He will think only about food. This thought will override all other thoughts. Pain has a similar effect of overriding all other thoughts but this is not a good advice to follow because it may lead to mental disorders. We need to take heed.
I liked very much, what you said about the construction work: it is not nearly as complicated and hard to understand if you take time to think over it. Similarly, thoughts are also not as complicated and hard to differentiate as they seem to be. You simply have to calm down and bring yourself to reason.
Each thought that you want to direct towards God calls for immense effort; and finally, you lose this thought because it calls for vigour and strength.
How can we minimise our losses? When you direct your thoughts to God, you are strained; and then you understand that you cannot bear the strain and that you have to relax. You should bring your losses during that period of relaxation to a possible minimum; your thoughts should be neutral. For instance, you might sit around, drink some tea or stay alone for some time. If you do that because you have nothing else to do, this is bad, this is a dead-end. But if you do that because you have to relax, this is not bad.
However, our enemy does not want to calm down even when you relax, he cannot tolerate you directing your thoughts to God. He wants to take revenge on you for this, which is why he will try to plant some wickedness even in these neutral thoughts because he knows your passions, your past with all its mistakes. He will strike you where your defence is the weakest, so you have to be ready for that.
This is why when you pray to God, you must understand that to every action there is always an equal and opposite reaction and that you are prone to failure at a certain point. An experienced military commander cannot always advance because he knows he may be surrounded. Things like that did happen during warfare. This is why you should back off to the prepared positions. It calls for your own reasoning, your spiritual intuition and your spiritual experience. You should understand when you are able to cope with the enemy's attack and when you are not.
Elder Sophrony was a great man of prayer, a true monk who received many spiritual gifts. He used to say that if an individual is capable of praying two hours with full and undivided attention, he has perfect prayer. Sometimes a person is in such a spiritual state when he does not pray but simply feels the grace and the power of God. One can experience such state for a day or even longer but this does not happen because of his own merit. When a person prays and concentrates his attention in order to be free from some emotions and reminiscences, this is a great stress, it's like shedding one's blood. You fight against the whole world and against yourself because your old man tries to stop you somehow.