Had girl friends, hobbied, married, divorced, hobbied.... found strippers willing to provide play for  pay, had gals in the champagne court, been drugged etc... in all that, never considered looking at a gal's face as an indicator of orgasm...
so?! How do I know?  Depends on the lady....

the ex Ms. Dude kinda had a high songlike wail she would let out.  That was NOT so pleasant, but at least it told me I was doing something right.

Most "get sweaty" with a 'dew' all over their skin.  This is usually accompanied by a change in their breathing pattern.  A couple have squirted on me.  One, provider in my town, goes into something that resembles an epileptic seizure, and her stomach muscles rhythmycally contract, while her eyes roll back into her head.   Several become supersensitive and cannot bear to be touched.  One dear sweet lady bucked and collapsed in a heap on the couch dance room floor....

So there are all kind of responses, me?  I look for the ones which cannot be faked (sweat, muscle contraction, supersensitivity, etc.).  I guess you could look at the face, but with all this over stuff going on, I may not be the best observer of subtle facial changes! lol

That might work for men because once you reach the point of no return, your orgasm is assured.  But for a woman orgasm takes concentration. So when I hit that moment when my head is tossed back, face is flushed and I am making who knows what kind of noise - I NEED to have my eyes closed.  That level of concentration and release of self-consciousness is what allows the orgasm to happen.  

But I agreee with everything else in the article.