The method used could be anything, for example scanning the brain and decoding brain waves into language and/or images using machine learning.

Experimentally, we should be able to do things like predict someone's internal monologue with only minor errors, or predict what answers someone will give to questions, with only minor errors. 

For example, if someone thinks of a number between 1-100 and we can consistently guess which number they're thinking of, this would be strong evidence for YES.

Reminder to read comments for additional clarifications. 