There are different levels of stuttering. I might not appear to stutter sometimes around the same people so it would be normal to think that they don't think I stutter unless nervous. Other people either know I stutter from extended conversations/time with me or I've/my parents have mentioned it. I wouldn't say anything to it unless you know FOR SURE. If it's right after a presentation or something where that's more common to be heard then I might nicely point it out. I wouldn't make a big deal out of it though.