Ha, hindsight is 20/20, as they say. I guess I would say that the voracious hiring, the dependency on other startups for revenue, the grand plans(an office in Londow and one in Singapore!), the hiring of high level executives that didn't seem to add value, the grand renovation (we had a custom built climbing wall in our building... was just got show), and later, the hamhandedness of the layoffs were all signs.
Of course it was a different world back then because the hardware and software were a much larger portion of spend for internet companies. Signs may be different now.
That said, "History may not repeat itself, but it does rhyme."