Measuring semantic complexity of a text

There is no uniform measure of semantic complexity that you could apply to any text regardless of genre, topic or speaker/listener/reader context. Even within newspaper articles you will find several genres and topic areas where a single measure is unlikely to suffice.

The term "semantic complexity" is used in the literature it generally refers to individual words and categories - e.g. in the context of aphasia.

Perhaps the best place to start your research would be the concept of readability which deals with text complexity in several dimensions by proxy. Another place to look is lexical density.

However, expect this to be a non-trivial problem if you want to go beyond the most simple of measures.