How to recognize that Librem 5 is Evergreen version?
Unless you received your phone several years ago, it’s an Evergreen.
The most helpful default assumption is “Evergreen”.
See also: What are the weight and dimensions of the Librem 5? for another way to investigate.
I don’t think Purism exactly made it easy to answer this question in a simple, definitive way.
Only Evergreen have 4500mAh battery?
I’m not in a position to say one way or the other. I have an Evergreen and I didn’t write the FAQ but the guy who did write the FAQ knows his stuff, so if he says so, I will trust him.
Of course in principle you could find a compatible battery and replace the supplied battery with one that has a different capacity - so I guess any statements made are applying to the Purism-supplied battery (which has the Purism name and logo on it).
Yes, it’s also slightly longer to fit that battery, see https://puri.sm/posts/librem-5-evergreen-update-mold-and-milestones/