We know that the Mitochondrial Eve (the closest common female ancestor of all living humans) lived 100,000 to 200,000 years ago and the Y-chromosomal Adam (closest common male ancestor of all living humans) 150,000 to 300,000 years ago. They likely never met. How is that possible? Doesn't matter for this question, but what does matter is that the scientific methods which were used to find this out could also be applied in your world to find out that both only lived 10,000 years ago.
There would also be a very small genetic diversity among humans when all would descend just from the genome of two people and that genome only had 10,000 years to get any new information through mutation.
By the way: According to this source, a single human couple might not be enough to repopulate the earth. A gene pool of at least a few dozen, better a few hundred, individuals is required for a stable population, otherwise the species will inevitably die out after a few generations (the big plothole in oh so many creation myths). But I don't want to kill your whole premise, so let's ignore this problem for now.
When the whole world population was reduced to two people, their descendants would likely be concentrated in a very small geographical region for several generations. Only when that area would start to suffer from overpopulation would they start to colonize areas further away. Humans would spread geographically, but this would be a very slow process. That would result in the oldest cultures all emerging in one region and subsequently younger cultures emerging further away.
It would be obvious that human culture emerged from just one point and then spread slowly over the whole planet.
When Ragnarök also destroyed all archeological evidence of existing civilizations, it would be notable that no remains of human civilization older than 10,000 years exist. When Ragnarök left ruins behind, it would be notable that all civilizations ceased to exist at exactly the same time, and that it then took several millennias until new civilizations emerged in the same area and that those had no cultural connections to those which were there before.
You said that animals and plant-life also suffered from Ragnarök and its aftermath. This would likely mean a mass-extinction event which caused several unrelated species to suddenly become extinct. We know that several such events happened in the history of Earth due to fossil evidence, and for most of these events there are theories (of varying acceptance) why exactly they happened.
A very interesting evidence for how climate has evolved on our planet is through ice core sampling from polar ice or glaciers. Every year a new layer of polar ice forms, and by looking at each layer we can tell how the climate was in that year. By analyzing particles contained in the ice we can also find atmospheric contaminations.
It is possible to retrieve core samples from hundreds of meters below the surface which provide us with information about the climate over 100,000 years ago.
The Ragnarök event would likely result in the ice layer for that year to be quite unusual.
It might not be obvious what exactly happened, but it would definitely be impossible to deny that some cataclysmic event happened exactly 10,000 years ago and that this event drastically decimated the human population to very few individuals at one location.
You said in a comment that "they disappear without a trace, along with all direct proof of them ever being there". When this premise is absolute, there is by definition no way to tell the difference between survivors and humans which were newly created at that moment.