Obviously a searcher's chances of finding something valuable or historic are greatly increased by looking in places where something has already been found. Thus it is we see artefact hunters targeting known archaeological sites. There are various means by which they go about locating them, reading the archaeological literature is one way, many reports show where the sites are. For those less patient in their "research", there are ready-made databases.
Here's one advertised:
Vignette: Metal Detectors Searcher CD-ROM is just one of many similar offers for metal detectorists in the UK allowing them to target known archaeological sites and empty them of collectable artefacts.Metal Detecting Sites and Finds
Working with Google Earth Placemarkers
Metal Detecting in Wales: Metal Detectors Searcher CD-ROM has over 833 sites and finds in WalesFindspots of Roman Coin Hoards.
Sites of Roman Villas, Forts, Buildings, Settlements, Cemeteries.
Sites of British Battlefields
Findspots of Bronze Age and Iron Age Artifacts.
Sites of Deserted Medieval Villages