Depends...
Liquid extract will get darker with time. (So will dry, but at a much slower rate).
If it is a can, make sure it isn't swollen. If it is, just chuck it. If it is dry, it should be dry still, and not totally clumped together. Stored properly, these will last for a long time.
Assuming the kit had dried yeast with it... throw it away! Yeast does have a limited shelf life. Just pick up a new pack of something dry to match the style.