A red flag of pseudohistory is somebody acting like nobody in history ever interacted with each other until Modern Times.
For an example of this, there's an episode of Ancient Aliens ("Decoding the Dragon Gods") where a guy talks about all these different languages that describe dragons with a word similar to "dragon." He claims that the only explanation for this is that they all came in contact with the same beings, because allegedly none of these people ever had contact with each other.
But he's wrong. The reason so many languages use a word similar to "dragon" (yes, even Swahili) is because the Romans had the word draco (derived from the Greek drakon), and Latin has had a major impact on languages all over, both directly and indirectly.
And I mean... just imagine claiming with a straight face that Europeans never interacted with each other or influenced each other's languages for thousands of years, not even in the medieval period. Like, holy shit. Even the most cursory look into history would disprove this.
So yeah, watch out for people claiming or implying that people didn't interact and affect each other's cultures until arbitrarily recently because that's a red flag.
(Also, if you find somebody making weird-sounding claims about languages, wiktionary.com and Online Etymology Dictionary are good places to look stuff up.)