I guess we can asume that you''re talking about older line. Generally, nylon has more stretch/elasticity. However, as it ages it gets stiffer and it becomes harder to tell - especially with less expensive "non-marine" cordage.
Can you provide a little more information - like why you are asking this question?
Chances are that,if it is double braid,it is dacron. Although, there are some manufacturers that make dock lines that are braided. Also, if the diameter is less than 7/16" or so, the lines are probably old dacron sheets, halyards or control lines. Of course, the size of your boat also matters. Its still pretty hard to generalize without seeing them.