As a rule of thumb, such pictures can have their main subject(s) easily extracted from the background and placed on other backgrounds. Backgrounds that don't "exist", exist as as an infinite empty void, aren't physically "lived in", or lacking in significant detail or iconic elements can be considered.
Immediately recognizable background patterns, like grid background or sunburst background should be tagged as such, but depending on its integration can be a candidate for simple (i.e. flat color or of low detail).
