You will need to be careful with this. you cannot be sure that what looks like nodding or even what sounds like "yeah" in a smaller voice means confirmation. "nodding" can be the beginning of an attempt to say another word. what sounds like "yeah" could be just a filler word or the first syllable of another word. people hallucinate a finished yes all the time and i cannot always correct them quick enough like "no i mean \[...\]". and if I correct them at a delayed time, like a few seconds later, there's a chance that they'll get mad at me and accuse me of being passive aggressive. And after more than a few seconds? it'll be too late. so I assume some stutterers might give up correcting. Let's say the stutterer usually has 10 seconds of block here and there during speech. If you ask "ketchup?" mid-sentence, you will need to wait at least 10 seconds more to be sure that what you heard is really a "yes." full stop or to see if you're hearing a continuation of their original sentence. It's simpler to just wait the first time.