from how they present themselves in video calls. Bad audio quality, bad framing (e.g. those people that occupy only the lower half of their screen), bad backgroubd removal (you don't have to show me your flat, but if the keying looks crappy, maybe you should have used a neutral wall instead), bad lighting (you don't have to do a 5 point lighting setup, but the image should not be noisy as fuck because it's much to dark or half of your face should not be severely overexposed),... all of that shows me that you don't care and don't sweat details and thus influences my motivation for doing business with you.
I do overlook glitches due to bad connections, though.