Well in the case where my former girlfriend was giving me a bj, I'd just say "Yeah babes I'm cumming." The best part about that was I would say it just as my first spurt was shooting out of my prick. You see, my girlfriend didn't like to perform bj's (honestly she was terrible at it so I often had to stroke myself while she sucked on me) because she didn't like the taste and texture. But, me being so perverted and enjoying the sensation if she was to stop I wouldn't end up enjoying it. So by the time she could react I'd already released a huge load and she'd either gag and spit or better yet swallow before she choked (and she hated swallowing more than performing a bj). But gotta say she was a trooper. I guess given the choice she'd more often than not choose the bj rather than the 3 hour fuck fest cause those marathon sessions had her trying to literally tap out from non stop orgasms.