You know it's fake because the company hasn't made you sit through 6 interviews with random panel guests then ghosted you for 3 month then low balled you and put a 24 hour expiry on the offer. Bonus credibility points for only giving half of the question during the whiteboard session only to casually remember remember at the very end while grading harshly for missing that.

If the offer hasn't been rescinded at least once it's probably a red flag as well.