We know that this natural disaster can cause a big damage. I have heard that we can identify such areas by some geographical signs, such as curved trees. Do we have any stronger detectable signs which we can observe easily?
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:
Terrestrial laser scanning (the ground-based version of Lidar) provides very precise mapping of surfaces. This has been used to look for changes in cliff faces and slopes that are precusors to failure.
I'm not sure if this is the scope you're looking for but a large portion of site analysis using GIS is to examine potential risks of natural disasters. Mass wasting risk maps based on topographical and soil characteristics as well as extreme precipitation events is a pretty standard research topic. Using fuzzy/boolean multi-criteria analysis, determining areas of potential landslide risk can be ascertained.
Some example methodologies for determining landslide risk in different locations:
In developed areas perhaps the most telling evidence of a developing landslide in its early stages are doors and windows that no longer open and close easily and cracks in pavements and curbs that line up to define the boundaries of the slide.