Previsual literally means "before seeing". Usually, it is applied to a means of detection of a phenomenon before its consequences are noticeable or irreversible. Previsual detection lies on the influence of the phenomenon on another indicators different from those than one intend to avoid.
Example
An example of previsual detection would be that of a plant stress (for instance lack of water or nutrients) by means of its influence on sun-induced fluorescence emission of light by the leaves (Chappelle et al., 1984).
References
Previsual Wikipedia(Text) CC BY-SA