Archeological dig sites are often identified by pure happenstance. Aso many finds are locate under current cities and towns, a site may be found when new construction begins.
A crew goes in and begins excavation for a new high-rise or townhouse development or office building or shopping mall or something else (such as a synagogue or funeral parlor) and notice bones or some other item which seems old. The project grinds to a halt and experts are brought in to determine if these are ancient relics or just a crime scene. (Although they could be both)
Other times it could start with a person just poking around in their yard or at the park or beach and locating something.
There are other identify methods as well.
Regarding recovery, it's a slow and tedious process which includes identifying and cataloging of each item found.
For more on archeological digs, visit this helpful website:
and this page in particular which mentions details on gatherment.