Collecting geologic evidence takes much effort in evaluation of landforms, sediment trails in valley systems.  There are a variety of disturbance regimes, especially in Tectonically active areas as landforms are modified, fractured, folded, sheared, shaked and forced into a position of instability.  Certainly excessive rainfall is often the driving force that "greases the skids" so to speak in overloading the weight and lubricating fractures or material discontinuities.  But there may be other disturbances that contribute, such as wildfires, high winds with sufficient force to fell or uproot trees, rivers overloaded and aggraded in confined landscapes that undercut the adjacent steep slopes, etc.  In other circumstances, channel degradation can undercut slopes as evidenced by nickpoints that contribute to slope or soil material instability.  To be able to definitively identify the exact cause may be difficult, but rainfall events are probably high on the list of factors producing "the last straw" that results in failure.  Collecting, reading and understanding the evidence takes a variety of skills and effort to piece it all together.    

Most of the time 'big' palaeo land slides are often associated with huge rainfall and earthquakes or both. So if you have a good  rain fall data or hiatorical earthquake data, you may try them to correlate with your land slide event. Uunfortunately,  these data records are not typically very old,  so there is a limit to it. The other way could be finding indirect evidences for "wet" periods. Having said that, finding exact cause of a land slide could be a daunting job.

@Mr.Hansen and Mr. Malik, Thank you for the response. I think i need to collect more imprints.