The phrase "ascertain a fact" is correct and usable in written English.
It can be used when you want to express the act of finding out or determining the truth of a particular piece of information.
Example: "The investigator's job is to ascertain a fact before making any conclusions about the case."
Alternatives: "determine a fact" or "establish a fact".
To ascertain the fact, a visual comparison was made between drainage network apparent in the topographical maps (published in 1972) and WorldView-II satellite imagery (acquired in 2010) followed by field verification.
Due diligence is the investigation that a reasonable person would make in good faith to ascertain the facts in a situation or to determine the truthfulness of assertions.
You might be the only human for miles, but you can't visually ascertain that fact as readily as you can in Montana.
Radium in Nature. Radium exists in minute proportion in every kind of soil and water; the extraordinary sensitiveness of the methods of analysis has made it possible to ascertain this fact.
We took these allegations very seriously and we immediately launched a full investigation to ascertain the facts.
"Once we ascertain the facts, the president will make an informed decision about how to respond," a White House official said.
"We want to make certain that they are aggressive enough to be able to ascertain the facts in the face of an adversary who in the past did everything in his power to hide the facts," Mr. Fleischer said.
Participants classified as clinical specialists were provided CPGs within their broad area of practice; but, we did not ascertain whether in fact the specific topic of the CPG was an area in which they actually did have knowledge or experience.
On the other hand, it must be remembered that it is an ascertained fact that the aborigines taboo a word on the death of anyone bearing that word as a proper name.
