Telluric currents are very low frequency electrical currents that are subterranean or submarine, that are, according to Wikipedia, the result of natural and/or human activity. Today we can detect them with modern instruments. But in the past, certain ancient peoples (notably the Celts) would build dolmens nearby or on sites with telluric current. These are sometimes referred to as ley lines amongst esoteric groups.
Unfortunately, I cannot find anything about how these electrical currents were detected, apart from new age blogs. How did the Celts do it?
(I apologize if this is a question more suited for AskScience!)
European megaliths (dating to largely from 6,500 to 3,500 BP) were erected in western Europe before the likely arrival of the Celtic languages. How the Celtic languages were or were not linked to arrival of any given people is another matter. Nevertheless, the Celts are often linked in many modern minds with spiritualism and mysticism, so their identity is often projected back in time to megaliths. Celtic speakers did not erect megaliths.
The Neolithic people who began erecting megaliths in western Europe left no written records. There is no evidence that they had any technology to detect subterranean electric currents. The question about whether such currents even exist and/or if they are coincidental with megalithic structures should be taken to /r/AskScience. For purposes here, we can assert with reasonable confidence that Neolithic people in western Europe did not have any technology related to electrical currents.