Front: Not necessarily announced aloud. Though it is pretty obvious because breathing changes, and motion slows down.
As for the back: you couldn’t miss it because vocal all the way through especially at the end; my thighs would also pop a watermelon if one was there.
On my own, no, but with my husband, yes, for sure. I’m sure he can tell but he loves to hear it so usually it will be something like ‘I’m gonna cum’, ‘I’m starting to cum’, ‘there it is’ or ‘I just came’ again. I’ll usually have 2-4 orgasms in a session, usually 2 on my own with my vibrator but when we are together it can be more.
Routinely, no, although MrsH will often prelude an orgasm with repeated shouts of ‘don’t stop!’. That said we’ve both talked over the years about how much of a turn on it his to hear the other announce they’re close or there, regardless of how obvious it is, so we try and work it in more these days.
I always do, even if I’m on my own, unless I’m testing someone’s willingness to swallow my spunk, I let the event speak for itself!
Lady partners: very rarely, but one of my fantastic ex-wives used to go bright red and start to shudder, so I always knew how to handle that to make it better.
Male partners: it’s generally “don’t stop” and sometimes “I’m coming”. Of course, if it’s oral you get more pre-cum as the actual event is about to happen.
We both do every time. That few seconds after she announces it is great just before she cums. I like to watch her face. 
