When you say the ‘quality of roasted coffee’, there’s 2 things that come to mind. First, the quality of the green. For this, if the roaster is buying high-quality screened coffee, then the bean/seed size should be reasonably uniform and we’ll assume for this forum that it has little defects and we’re not talking about a blend. The second thing (and the one I think you’re getting at here), is if you can tell the quality of roast from visual cues. Similar to the comments from #the2nicos below, taste is king, but there are a couple of indicators that lend to the expectation of a decent roast. 1. Are the beans a relatively even colour and ‘plump’ or full without much wrinkling? and 2. How do they smell? Are they fragrant and pleasant or are they a little grassy still?
From my personal experience, there’s a couple of other things I like to check if I can. The first is the colour of the inner chaff still clinging to the ‘seam’ of the bean. If it’s light brown or darker and the seam is slightly open, then I think this is a good thing. If the chaff is still a little greyish-looking and the seam is tightly closed, then it’s in my head already that there’s likely some underdevelopment. Sidebar: this is much more common and to the point of being almost a problem among ‘specialty’ roasters. The other thing I check is how brittle the beans are; can they easily be broken with my fingers? If so, then this too s a good sign. If I have to dig my thumb nails in to rip them apart, then I assume (either rightly or wrongly) that the batch is again underdeveloped. There’s obviously exceptions here, but these are the cues I like to use.