I've had servers and even fast food workers try to finish my sentences for me because they hear the first letter of the word I'm trying to say and can usually figure out what I'm trying to order. As far as how they approach it. I'd say just either keep eye contact but without any reaction, especially if it's frustration or a grin. (Not saying you would do that.) But at least in my case, if they look at me as if they are simply listening to me talk but are waiting for me to get through it, it makes me feel more comfortable versus them awkwardly looking away or smiling. And when they do finish my sentences or try to guess what I'm trying to say, if they say it in like a soft & questionable tone, it doesn't come across as "DUDE COME ON". If that makes any sense.