There's a guy I've been playing with for a number of years (a very sexy builder) and I still sometimes miss the cues for when he's going to cum, mainly because he doesn't make a sound.
It's a bdsm meet, with him restrained to the bed, getting his big cock edged and having his nipples worked over.
For judging when he might be getting close, I have to watch his face - when the orgasm is starting to build, he holds his breath and clenches his (manly stubbled) jaw - and watch the rest of his body - for him tensing his (impressive, furry) stomach muscles and straightening and tensing his (muscular) thighs.
But with him not making any sound, it's difficult to know just how close he is - sometimes I want to get him close but not let him cum and there's been a couple of ruined orgasms when I haven't managed to stop in time.
His cock does definitely swell as well, but that's difficult to reliably read too, as his cock is usually already at a hefty swell thanks to being bound and tied with a shoelace.