It all depends on several things.If you are coloring your hair to cover gray and you mix a developer with it, it should be ok as long as it looks like it always does. Developer will loose its strength gradually over time and even unopened color will oxidize after a period of time. Color that is truely bad will smell even worse than normal .If in doubt, throw it out. It will cost you more money in the long run if it doesnt give you the same results as always.

there should be a date on it