Everything counts except odd bits of dialog, e.g. Zefram Cochrane being referred to as "of Alpha Centauri", or the Eugenics Wars being referred to as in the 1990s.
Why don't they count? How do you know Zef didn't go to live on A Centauri later on? And How do you know that in the Star Trek timeline there were no Eugenics wars?
Yes, you can come up with explanations. But my point is that if some odd bit of dialog contradicts events actually seen later on, then the bit of dialog is what I'd drop. I'd throw out the reference to Alpha Centauri, which had no actual effect on plot, rather than everything in First Contact with Zefram Cochrane. And I wouldn't feel compelled to make their 20th century so different from ours just because of a line about the Eugenics Wars in the 1990s.