[you]: Knowing? How can any human KNOW what the other side in a conflict will do?
Here is what Governor Pickens told Ward Hill Lamon, Lincoln's personal representative, in March. Lamon assured the Governor that Fort Sumter was to be evacuated [Source: "Recollections of Abraham Lincoln" by Ward Hill Lamon, my bold below]:
My interview with Governor Pickens was, to me, a memorable one. After saying to him what President Lincoln had directed me to say, a general discusion took place. [The Governor said the following at the meeting] Nothing can prevent war except the acquiescence of the President of the United States in secession and his unalterable resolve not to attempt any reinforcement of the Southern forts. ... Let your President attempt to reinforce Sumter and the tocsin of war will be sounded from every hill-top and valley in the South.
The Governor was quite agreeable to having the fort evacuated. The Governor's account included the following [my bold below]:
In a very few days after, another confidential agent, Colonel Lamon, was sent by the President, who informed me that he had come to try and arrange for the removal of the garrison, and, when he returned from the fort, asked if a war vessel could not be allowed to remove them. I replied, that no war vessel could be allowed to enter the harbor on any terms. He said he believed Major Anderson preferred an ordinary steamer, and I agreed that the garrison might be thus removed. He said he hoped to return in a very few days for that purpose.
There were earlier indications of the attitude of South Carolinians about the forts. From Major Anderson to the Adjutant-General of the Army in Washington on December 1, 1860 [From the Official Records; my bold again]:
Captain Seymour, just returned from the city, reports that the excitement there is very great. Col. E. B. White and other gentlemen, with whom he conversed, stated that the people of Charleston would not allow another man or any kind of stores to be landed at or for these forts. They say that anything which indicates a determination on the part of the General Government to act with an unusual degree of vigor in putting these works in a better state of defense will be regarded as an act of aggression, and will, as well as either of the other acts mentioned above, cause an attack to be made on this fort [Fort Moultrie].
Had the South previously rejected a resupply mission with force? Yes. But people change their minds, and what “the people of Charleston” wanted might give way to what was good for the Confederacy as a whole.
So no, in a war, until you actually TAKE an action, you do not KNOW how the other side will respond. In any case, the South had a choice. They could start the war, or insist the North start it. And the SOUTH CHOSE to open fire on Fort Sumter. No one “made them” do it! They were not defending themselves from a Union attack. It was NOT self-defense.
After all, Lincoln BELIEVED that the border states would stay neutral if the South fired first, and his belief was proven wrong. He lost Virginia, and he thought Virginia might stay in the Union.
No one KNEW anything. That is the way a war works. And Lincoln, with one month in office, was receiving a lot of conflicting advice. Conflicting advice is another way of saying, “No one KNEW”. If they knew, the advice would be uniform.
So, like any natural-born Democrat, SC Gov. Pickens tells the Union: if your face gets in the way of my fist, it's all your fault.