Well, you probably know your study area and can estimate where displacement is reliable and where not.
If the fringes show nice patterns, there is nothing wrong. But if you see a trend in your data superimposing the fringes or have some noisy smaller areas, these are probably not of use.
Always look at coherence and mask out low coherence areas to exclude unreliable pixels. Have a look at this example: Subsidence map in 3d view