How long after mixing do you figure 900ml of stock XTOL would last in a tightly capped dark glass 1 liter bottle stored at about 22-23C?

Usually I use up my developer stock solutions pretty quick but in this case I mixed up a batch of XTOL in mid July and in the past two months I've only used 100ml from the 1L bottle. I'm doing some new work now and wondering if it's likely still good. And if there are any ways to tell whether XTOL has gone bad (color change etc).