As he climbed up a ladder to get the money from above the sefarim shafer (bookcase), he thought, "Why should I give all this money to one family? Wouldn't it be better to divide the money among twenty families?" But then he thought, "Until now, I planned to keep the money for myself, and I never considered dividing it among twenty poor people. Why did this idea come to my mind specifically when I was about to perform a mitzvah of help an almanah? This is a sign that the thought is the yetzer hara," and he gave the money to the widow.
When his wife came home and found out that he gave the money to the widow, she asked him, "Wouldn't it be better to distribute the money among twenty poor people? Why did you give it all to one widow?" He replied, "I had the same thought when I climbed the ladder to get the money, but then I realized that I didn't consider giving the money to tzedakah until then, and I realized that this thought was from the yetzer hara, so I gave her the money."
Another counsel is asking yourself what you would advise another person to do. Sometimes, people know better what others should do than what they themselves should. Removing yourself and your biases from the picture will help you make a better decision.
There's another counsel to know whether your thoughts are from the yetzer hara or the yetzer tov. If the thought keeps returning to your mind, that is a sign that the yetzer hara is talking. An indication of this concept from the Torah is the story of the wife of Potiphar. Aishes Potifar told Yosef that she saw in the stars that she was destined to bear a child from Yosef. Her intentions were l'shem shamayim (as Rashi tells us). But Yosef perceived that it would be an aveirah. How did Yosef know that? The pasuk (Bereishis 39:10) states, אליה שמע ולא יום יום יוסף אל כדברה ויהי עמה להיות אצלה לשכב, "It came about when she spoke to Yosef day in and day out, that he did not obey her..."
She spoke to Yosef every day about it. The persistence was a sign of the yetzer hara. This indicated to Yosef that it was wrong.
There's a counsel from the Alshich HaKadosh of what one should do when he doesn't know whether the urge in him is the yetzer hara or the yetzer tov. He should do just a bit and see how he feels after that. For example, if he is motivated to give a million dollars to tzedakah but isn't sure that this is what Hashem wants from him, he should begin by giving a hundred thousand dollars to tzedakah and see how he feels after that. That will help him know whether this deed is Hashem's will or not.
The Alshich and the Malbim say that Dovid HaMelech used this technique when he found Shaul sleeping in the cave. He wasn't certain whether he should kill Shaul or not, so he cut off a corner of Shaul's clothing. The Navi says (Shmuel 1, 24:6) אחרי ויהי ויאמר ,לשאול אשר כנף את כרת אשר על אתו דוד לב ויך כן למשיח לאדני הזה הדבר את אעשה אם 'מה לי חלילה לאנשיו הוא 'ה משיח כי בו ידי לשלח 'ה, "And it was afterwards, that Dovid's heart smote him, for he had severed the corner of Shaul's clothes. [Dovid] said to his men, 'Hashem forbid it for me, that I should do this thing to my master...to stretch forth my hand upon him, for he is Hashem's anointed.'"
The Malbim writes, "Dovid was...unsure whether Hashem wants Shaul to be killed by the person he was pursuing. So, he tested it by cutting off the edge of his coat. This action was a drop of the deed to rebel against him and desecrate the king's clothes. And behold, Dovid immediately felt אותו דוד לב ויך because the heart of the perfect people feels regret when they do something wrong. This is how Dovid knew that Shaul's kingship wasn't completed yet. He was afraid of the king and realized he sinned by cutting off the edge of his clothes. And all the more so, if he were to smite Shaul himself."
Similarly, the Alshich writes, "Dovid said, 'If my heart beats from regret for cutting off the corner of his cloak, certainly it will be bad in Hashem's eyes if I will smite Shaul, himself.'"
