How can you tell how old the line on the shelf is or when it was made ??

You just have to pull it off the shelf and see if the line snaps when you yank on it.

I don't know of a code to check for freshness although i have heard that some line manufactures are considering adding a "freshness date" to there product....I buy my line at a place where they have lots of business like BPS ..I wont buy a line that looks Dull or is on clearance at walmart that is normally a sign of it bieng old very old ...I've found this doesn't matter on Super lines made from "Spectra" like Powerpro ect...I have used the same spectra line for several years with no problems but it's not suited for some of the fishing i do..Spectra is worth extra money if it will work for you...
As for mono, even if the line seems to hold the rated strength when jerking on it or knot testing it, the real test is after getting it wet or with it staying wet after fishing for a while the knot in old line will fail and break easy ...line is one of the most reasonable priced tools you can buy to catch fish don't skimp to save a few dollars...

TIP...never let the store spool your reel for you theres a good chance that the bulk spool is "OLD" line...It takes along time to sell 3000 yds.