DH = dry hole. And yes they do have the technology now (actually for quite some time) that will tell them if they have found hydrocarbons as they drill. So they immediately know if they have found hydrocarbons but it takes them awhile to figure out how economical the discovery is, if there is one or multiple zones, where to drill next.....
With one well they will never know the extent but they get a good idea once they test the well (flow it)and monitor it for a short period of time.