I am a few years away from pushing probes into the soil and getting in amongst drainage with a spade Boab, sorry.
Some folk can just eyeball a piece of land and tell where the drains will run, that ain`t me. Other than that there tends to be a fair bit of probing and hole digging until the line of the drain is discovered. A wet patch may be a red hering for location unless you can see water rising.
The foundations of the 50`s part of my house were laid right through a field drain line with no alternative routing considered. If your issue is not something that has been going on for years, and you may have had recent excavation in your area, perhaps for upgrading of telecoms or similar then that could be a logical starting point..?
