Boy, did the OP get a lot of incorrect, condescending and irrelevant answers.
Yes, it is possible to check as to whether or not a PDF has been flattened, or is flattened, or whatever terminology quibble the responders prefer.
One method/program (or whatever terminology quibble the responders prefer) is called pre-flight and there are others.
If it were impossible to check, then on-demand publishers like CreateSpace would not be able to check, which they do.
Techno-babble is great for impressing people and trying to get them to jump through hoops (especially when accompanied by condescending tone) but is otherwise useless.