This is a good question. Unfortunately, this isn't always easy to tell as most toilets are designed with an eye towards durability rather than age identification. However, certain design features can give away the age of your toilet. The main clue we can point you in is how low or high current model numbers are when compared to previous models.
For example, if a particular American Standard toilet model was introduced 10 years ago and it retains the same number today, then chances are that it has been discontinued and a newer version has taken its place. If nothing else works, then the best way to tell how old your toilet is is by looking at when it was made. To do this, you will need to look at the manufacturing date (often stamped into the bottom of the tank). This might be in a slightly different place, depending on the toilet brand you've gone for, such as Kohler, American Standard or even Toto toilets.