This is not really answerable. We know he was smart and incredibly competent. He also was not a 100% peasant farmer but from an Ashigaru family, and did for a time go get employed under the Imagawa before coming back to seek employment under the Oda. But, as with lots of historical records all over the world, we're only told what he did, sometimes why, not how he knew to do them. Unless you count the Taikōki's apocryphal account of the man recommending Hideyoshi to Nobunaga by comparing him with Han Xin and Zhang Liang, two important men to the founding of the Han Dynasty in China.

Having said that though, we have to recognize that some of his exploits are likely legends created later. For instance, about his legendary feat of constructing the one-night castle, all Chronicles of Oda Nobunaga say about Sunomata Castle is:

In Eiroku 4 [1561], the Year of Metal Junior and the Bird, in the first decade of the Fifth Month, Nobunaga crossed the wide expanse of the Kiso and Hida rivers and forced his way into western Mino. After setting fire to several localities and settlements in the area, he gave orders to establish a strongpoint at Sunomata. While he was encamped there, On the 23rd of the Fifth Month, the enemy advanced in full force from Inokuchi and deployed at the village of Jūshijō. Accordingly, Nobunaga rushed from Sunomata to engage the enemy with his light infantry. In the morning battle, a friendly unit led by a younger brother of Zuiun’an pulled back when its commander was killed. Having gained momentum, the enemy pushed on to North Karumi and formed up facing west. Nobunaga reconnoitered the surroundings on horseback and redeployed to West Karumi, disposing his forces toward the east in front of an old shrine. As the light infantry skirmished, darkness approached. With Makimura Gyūsuke at their head, the enemy charged, only to be repelled. Here Ikeda Shōzaburō and Sassa Kura no Suke, acting together, killed and decapitated Inaba Mataemon. As the fighting was protracted into the evening, there were those, on the one hand, who bent to the pressure and fled, while there were also those, on the other hand, who stood tall and kept fighting. During the night, the enemy encamped.
Nobunaga remained at his field headquarters until the first light of day. The morning of the 24th, he returned to his fortified camp at Sunomata, having abandoned it soon afterwards.

If Sunomata was integral to the taking of Inabayama in 1566, surely more would've been mentioned. The Taikōki on the other hand does say that in 1566 Hideyoshi spending about two months gathering the building materials for a wooden fortress at Komaki, setting out from Komaki on the fourth of the ninth month, arriving at the northern river and crossing into Mino by the fifth and setting up the outer walls, making a stronger castle by the seventh or eighth, and then holding it until relieved. But, even when we already know the author likes to exaggerate, neither was the castle built in a day, nor was it at Sunomata, but somewhere along the Kiso river less than 10km north of Komaki, rather than Sunomata over 20km west of Komaki. This would mean it had nothing to do with Inabayama, and indeed the Taikōki doesn't say it does.

We must also recognize that his work at diplomacy when he was under Nobunaga was easier than often assumed because the context is not taken into account. Since before the Mino campaigns, the Saitō clan was in deep internal turmoil, and that is in fact what lead to the campaigns. It would be far easier to convince deeply divided Saitō retainers to switch sides than if they were united to resist an outside invader. After Nobunaga gets to Kyotō, he was so far ahead of everyone in terms of population and economic power, the diplomacy was really just convincing people to switch to the winning side in return for recognition and security of their current land holdings. These diplomacy was as much Nobunaga's talents as they were any of his individual vassals he sent on diplomacy, of which Hideyoshi was only one of them. The same could be said about his military exploits. Once the first coalition against him failed, Nobunaga gave Hideyoshi, as he did all his other generals, resources far beyond what most of their adversaries could muster.

On the other hand, we must recognize that, even if Hideyoshi was given a good hand, he still played it incredibly well. He did rise incredibly rapidly under Nobunaga, and one of Nobunaga's most defining characteristics is he promoting people purely on talent while most other daimyō still clung to using fudai families, so to have risen from an ashigaru/low samurai to one of the leading men under Nobunaga in less than 15 years must mean Hideyoshi was very talented. His flooding of Takamatsu (although people seem to forget that it did not actually do much, though eventually it might have), diplomacy with the Mori (though he had a bit of luck), and rapid countermarch, and then his string of political maneuvering to become the de-facto successor to Nobunaga, all demonstrate that even without Nobunaga's leadership he was both incredibly talented and experienced. He outmaneuvered all the old Oda leadership at the Kiyosu meeting. He convincing his old friend Maeda Toshiie to abandon the battlefield at Shizugatake gave him the victory over Shibata Katsuie. And when he couldn't beat Tokugawa Ieyasu militarily at Komaki, he beat him with diplomacy. He was also sharp enough to realize while he did not have the authority, control, and perhaps military might to take Japan by conquest, ala Nobunaga, he had enough advantages to do so via diplomacy, which he did.

Personally, I would say that Hideyoshi was not a genius in that in all, or almost all, the things he did successfully the odds were in his favour. However, just in that alone would mark him as incredibly competent in knowing the best moves to make and when to make them. This is, after all, the Sengoku, when an incredibly powerful daimyō clan could lose everything in the blink of an eye.

The best English source on Hideyoshi is Mary Berry's Hideyoshi. It is unfortunately slightly outdated, and pretty much flies over his career under Nobunaga. But it does a good job of examining the unification under Hideyoshi.