All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah.
First: How did Adam (peace be upon him) see Iblis?
Allah, the Exalted, informed us that Iblis whispered to Adam and Hawwa (peace be upon them). He says:
“And he swore to them, ‘Indeed, I am to you from among the sincere advisors.’” [Al-A'raf: 21]
This indicates that Iblis addressed them directly, either by appearing before them or by remaining concealed within something else.
However, neither the Qur’an nor the Sunnah specifies how this happened. Therefore, we should refrain from asserting either possibility with certainty and leave knowledge of the matter to Allah, the Exalted.
As for the story that Iblis entered Paradise inside a serpent, it has been mentioned in some Israelite reports transmitted by Al-Tabari and others. This is something that is possible, but no authentic hadith from the Prophet ﷺ establishes it. Therefore, we neither affirm nor deny it.
Second: Can humans see the jinn?
The basic principle is that the jinn can see humans, while humans cannot see them, as Allah, the Exalted, says:
“Indeed, he sees you, he and his tribe, from where you do not see them.” [Al-A'raf: 27]
However, this does not mean that seeing the jinn is absolutely impossible. Some people may see them under certain circumstances, and it is authentically established that the Prophet ﷺ saw Satan and spoke to him.
Among the evidence is his statement ﷺ:
“Indeed, an 'ifrit from among the jinn came upon me last night, intending to interrupt my prayer, but Allah enabled me to overcome him...” Narrated by Al-Bukhari.
Ibn Taymiyyah (may Allah have mercy on him) mentioned that some righteous people and others may see the jinn, but they do not see them at all times.
Scholars have offered two explanations to reconcile the verse with the hadiths:
1. The verse describes what generally happens, namely that humans do not see the jinn. It does not rule out the possibility of seeing them on certain occasions.
2. The verse denies our ability to see them at the time when they see us, but it does not deny that we may see them at other times.
Some scholars held that seeing the jinn in their original forms is a special privilege granted to the prophets, and that others can only see them when they assume different forms.
Third: Did a war take place between humans and the jinn?
There is nothing in the Noble Qur’an or the Prophetic Sunnah that establishes that a war took place between humans and the jinn. Stories circulated about such an event are closer to myths and legends, or to Israelite reports for which no evidence has been established in our religion.
One should not become preoccupied with such stories. What matters is that a person recognizes Satan's enmity toward them and remains cautious of his whispers and attempts to lead them astray.
Allah, the Exalted, says:
“Indeed, Satan is an enemy to you, so take him as an enemy.” [Fatir: 6]
Conclusion: Seeing the jinn is possible under certain circumstances, but it is not the general rule. There is no authentic evidence establishing that a war took place between humans and the jinn. Likewise, no authentic hadith establishes the story that Iblis entered Paradise inside a serpent.
And Allah knows best.