Many makeup products, and not just "natural/organic" ones, have a symbol of an open tin with "##m" (or something similar) written inside it -- i.e., "This product is good for (length of time, in months) after opening it.", somewhere on the product label or packaging. It's a pretty good guideline to follow, IMO. But yes, mascara has the shortest shelf life of all -- three to six months. Other products you can tell they've gone bad in other ways -- if they start to smell rancid, if the texture changes, if it's a pressed powder product gone dry, if it's a liquid foundation that's started to separate (this happened to me recently), that sort of thing. (Powders do last longer than creams and liquids, though.)