We bought some land adjoining ours a few years ago, some very boggy areas, I would stamp footprints about and watch which direction the water filled them from, worked back to the source. An area which has been boggy for at least 20 years (welly-deep and lost my welly more than once, lucky I didn't stand in the bit where a spade went down to the handle!), is now terra firma, dug it by hand. Even found a small stone trough!
Trouble is, we find it addictive, we have enough to do without digging into more muddy patches and working back up the line to find problems, but we just HAVE to keep going back an doing a little bit more.
Normally flat stones across top of rough stone channels, but our last session unearthed very small clay pipes. Sadly these old drains were not built to cope with todays heavy tractors and machinery, which I think is cause of most of our problems.