Antonym of "assume" or; a synonym of "assume" and its antonym

Using assume in the sense that you are is the equivalent of its fourth sense from Merriam-Webster:

4 : to take as granted or true : SUPPOSE
// I assume he'll be there.

In other words, you simply accept something as fact. Even if you don't necessarily believe it to be true, you act as if it were true.

While forget is an interesting candidate for its opposite, I don't think it's as accurate as something else.

When you assume something, you are accepting it without question.

Therefore, it seems reasonable that question would be the antonym:

[Merriam-Webster]

1 : to ask a question of or about
2 : to interrogate intensively : CROSS-EXAMINE
3a : DOUBT, DISPUTE
b : to subject to analysis : EXAMINE

So, you might say something like the following:

On the contrary! Rather than assume those facts, I urge you to question them.

Or:

I question the truth of his statement.

Or in your specific case:

I question if X is actually zero.

Depending on the specific context and your intent, a synonym of question (the words in capitals in the definition) could also be used.

What about discount? You would have to change the "is" to "being":

discount X being zero

The Cambridge Dictionary defines the verb "discount" as:

to decide that something or someone is not worth considering or giving attention

I do wonder if it's strong enough to act as a true antonym of "assume" in this context, though.