...but in the heat of the moment wouldn't your actions and words let her know that the time has come? viz; "OK babe here it cums...oh yes...oh yes!"  
Even if you know she enjoys CIM I would still ask her that it's the way that you want to finish and, " Is that OK?" The answer I always get is, "of course!" or "hmm mhmmm".
I wouldn't cum in her mouth if I was unsure if it's on her menu or if I hadn't asked her first.

Even if she offers CIM, it's just polite.

It is nice you are being so thoughtful about this, though I wouldn't worry *too* hard or feel apologetic for cumming, I mean that is an expected outcome of what you are already doing right? If the situation seems like you should let her know (probably most of the time unless it's already clearly established?), you can make it hot with however you say it!! I guess some people are more vocal than others (like me, eek!) but all the feedback involved with "oh my god I am coming, I'm coming I'm coming!) is both super fun, practical, and sexy AND lends itself to consensual choice regarding taking it in ones mouth or not

But if you are unsure then yes, let her know what's about to happen.

On a golf course...

it's always appropriate to yell "FORE" if you think your shot may strike anyone in the direction of the shot's flight