How to measure visual salience quantitatively
The duration of eye fixation has been used in  both infants and adults to measure locus of attention. This can be achieved using an eye-tracker. A more primitive and less exact way is to manually count the frames of a video recording of subjects. The frames per second (FPS) would be a limiting factor. In addition, the item that was first observed is the more salient one, provided that there is no secondary bias in the task (i.e. the objects should be an equal distance apart or the distance could be randomized).

For visual search and serial search tasks, responses are gathered and individual response curves (reciever operator characteristics, ROC) for participants and conditions can be computed using signal detection theory (SDT). The discrimination index d prime (d') can then be calculated only if two explicit responses are required exist.

Another option is to vary the presentation time systematically in order to quantify processing speed, but this requires a more computational approach.