Covers that are dated, of course, pose no problem, nor sets that are known to have been issued in a certain year, but, for the vast majority of covers, unless you’re ready to make a career of studying covers, the best you can hope for is narrowing down your guess to within a factual range of years.

The length of the cover, the width of the striker, the use of certain types of footers, all are indicators of age. Still, it's possible to narrow the general time period that a cover comes from to a somewhat more precise range of years.

 For example, if the cover you’re trying to date is a front-striker, you know at least it’s pre-1979 (the last front-strikers appeared in 1978).  

Also, for most of the manufacturers, we know the years of operation.

The only general dating guide, and by far the most comprehensive ever done, is the MM Dating Guide. This guide lists the known range of dates for specific manumarks. As ongoing research reveals more information, the guide is normally updated annually. It currently runs 119 pp. and lists thousands of different manumarks.