I guess the question is do you want to know when the well in use now was built or when the water source was first used?

The most commonly used method is shouting down them at the top of your lungs, then counting the echoes. Each echo is roughly equivalent to a hundred years.

Not sure how kosher that technique is though. ;)

Just as a side issue, I wonder how many wells have been deepend over the years. If the water table has dropped like they suggest then there must have been quite a few. Presumably when first dug they only reached water level and just beyond!

You'de like the folklore relating to wells that move too, tjj, though one assumes these are spring fed wells. Usually it's because someone went against some custom or taboo practiced at the well/spring and it got offended and moved elsewhere, though in reality it's probably more to do with the water table changing with better drainage. A lot of wells seen on 1800's maps just won't exist anymore for this reason, if none other.

Not sure if this has been mentioned before but a fine list of wells in Angus .
http://www.fife.50megs.com/scots-folklore-magic-wells.htm