How to know the quality of drinking water prior to digging a borewell?

Is there any new technology to know about groundwater quality before digging a borewell? I mean is there any possibility to know that groundwater is sweet or not prior to digging? Please let me know, we have dug borewells three time about 25ft deep where we found salty water, I want to know that if we go more deeper like 250-300ft, would we find sweet drinking water ?