A good mustard never goes bad, it just fades away. Because of its high acidity, a mustard will not spoil or turn rancid, but will simply become weaker and lose heat gradually, becoming a shadow of its former self. This is because as mustard gets older, liquid is forced out of it by a gas that has formed. This same liquid, as it escapes from the mustard, forms a dark crust at the lid opening.
A general rule of thumb for mustard's shelf life is 6 months, even if it has not been opened. Unfortunately, much of the mustard imported into the United States has spent almost that much time getting here from its country of origin, not to mention the time it has spent on the grocer's shelf.
(Because mustard is such an inexpensive and showy way of filling a gourmet shop, many store owners order much more than can sell quickly. Turnover is in the range of four times per year, on the average. Therefore, one should be on the lookout for dust on the jars.)
To keep mustard in top shape, it must be refrigerated after opening. If you have a large family who loves the stuff, buy mustard in big jars, but if you go through a jar slowly, buy a small amount at a time. (Keep in mind the famous American condiment manufacturer who remarked that his fortune was made not on what people ate, but on what they left behind on their plates.)
To keep a jar of mustard especially fresh after opening, cut a thin slice of lemon and place it over the mustard before replacing the lid. Or pour a thin layer of oil over the mustard to keep out the air and prevent oxidation.
It's not a good idea to transfer mustard to a jar that is not glazed on the inside. Because mustard is so acidic, you risk lead poisoning by doing so.
When judging a mustard, it should not smell "eggy" when opened. You should not be able to taste the individual ingredients, such as salt or vinegar, but it should have a clean, smooth, complex flavor.
The ratio of liquid to mustard seed will determine a mustard's thinness. The thinner the mustard, the cheaper it is. American companies sometimes use flour, starch or other thickeners to mask a runny mustard.
On the other hand, a mustard without emulsifiers has a chance of separating and might need stirring when the jar is opened. Separation is not a sign of a bad mustard but a good one--a mustard that is perfectly homogenized probably contains emulsifiers.