There are some uncertainties in your problem:
- How noisy is the data? Is there variation of intensity within the plateau?
- Do you know the expected length of the plateau?
For 1 you could remove the noise by applying a filter, a small low pass filter would remove noise, but at the same time could modify the edges of the plateau. Thus a median filter could provide you with better results.
For 2, if you do know the length, you could use a moving filter checking for the variation within a range of values, if the standard deviation of, say 10,20,30 elements is below a certain value, then you know that you are in the plateau.