Symptoms of lens separation: there is (virtually) no reflection at a cemented glass/glass surface, but if the glue separates, about 8% of the light will be reflected back from that area (4% from each of the 2 glass surfaces that separated). This is invisible when looking THROUGH the lens, but when looking into it with bright light behind you, the separated spot looks bright and silvery. This does not affect the sharpness of the lens; it does reduce contrast somewhat in that part of the lens, especially in bright light; a lens hood will help. It will get gradually worse over time. The cosmetic effect is greater than the photographic effect.
Black & brass or gray patches around the edges: The ground edges of the lens elements are painted black during manufacture, again to prevent stray reflections and increase contrast. With age, this paint often begins to flake off or just separate slightly from the glass. Where the paint has separated from the glass, the glass surface will (usually) appear light gray like a groundglass surface, and where the paint is still bonded to the glass it will appear black, so you may see some blotchiness as this progresses. The effect of this is not too different from separation, it introduces stray light into the lens and reduces contrast. If the barrel is brass, you might see some of the brass color through this edge area of the lens. This is generally less serious of a problem than separation, mainly because it's fixable with some black paint. The effect is minor in terms of imaging, and can be improved by using a lens hood.
