How to identify a landslide area early?
Curved trees are a sure sign of movement of surface. Special geomorphic shapes are also present in a slowly developing landslide. At the top of the landslide cracks can start to show, at the foot you can find "bumps" in the soil. Water will have difficulties to drain at the foot of the landslide. 

Here are a few examples of literature that should be something to start with:


http://ny.water.usgs.gov/pubs/jrn/ny0155/jrn02-r20500g.pdf
http://www.ags-hk.org/notes/09/Geomorphology_and_Landslide_Hazard_Models_Steve_Parry.pdf
http://www.adpc.net/casita/pdf/01-Geomorphic-mapping.pdf