i used to have the same problem, so now i keep a sharpie type pen by the fridge, and write the date on the carton when i open it. if you prefer, you could count ahead 7 days and write your own 'use by' date on there instead.
its probably darker at the end of the carton cos of sediment- maybe a bit thicker too- the heavier stuff sinks to the bottom. shaking it every time you use it can help avoid that.
yep, soyamilk can go bad and hurt your tummy. once opened, air gets in, and bacteria start to grow. it might well look and smell a bit funny- curdled, chunky, smell 'off', etc, but its not quite like milk dairy does- that seems to turn really quickly, to me- one minute its fine, the next, foul.
i've seen some soya milk that was reeaaaallly past it, it split into a watery liquid, and thick, just solid, (puddingy/silken tofu consistancy really) beige gacky chunks, and a nice layer of mould on it. was really nice. or not. blargh. but that took a month or so to happen, so its maybe not something you'll have the pleasure of seeing for yourself.