One of the big mysteries for people outside of archaeology is how we are able to locate archaeological sites that have not already been exposed in some manner. There are many key attributes that we look for when considering what makes a good place to live and therefore create a site in the past. Most of these attributes are the same things that draw people to specific locations today.
Some of the characteristics for a good place to live are access to potable water, food (plant or animal), suitable terrain (flat, level, well-drained) and ease of access. Generally, in the warm weather months, this means a nice floodplain or lake side locale. Often, the trick can be identifying these locations as they were in the past.
As we know, streams often meander, meaning that where the watercourse is today may not be where it was in the past. And lake/marine levels raise and lower depending on many factors (melting glaciers, subsidence, spring flood etc.). Being able to identify old, stranded beaches or other key landforms in the middle of a forest, far away from any current watercourse is in part, what professional field archaeologists work towards locating.
Usually, the quickest and easiest methods for identifying these key landforms is by studying topographical/geological maps or modern and historical aerial photographs. With an experienced eye, interpreting the landscape and looking for relic beaches or other areas of archaeological interest is the first step in finding sites.
Once a site is found, non-destructive techniques may be used to quickly map the limits of the site (reducing the amount of excavation) and to identify areas of interest (hearths, foundations, pits, intense use). Engaging an experienced and qualified archaeological geophysicist that will use methods like magnetometry, resistivity and ground penetrating radar; much more information can be extracted from a site than conventional excavation alone and can help to reduce the cost associated with a lengthy stay in the field.As always, follow-up excavation is required in order to ground truth the geophysical results.
Depending on the needs of the client or the research, varying amounts of additional excavation may occur in order to completely remove the site (and as much information as possible) before development or to identify key pieces of information (age, function, size). There are also numerous in-field techniques utilising advanced technology and other disciplines that may add much more information to whole picture of what went on at the site in the past.