Head nodding
Just because someone looks you in the eye and nods her head – it doesn't necessarily mean that she agrees with you. Just because a person doesn't voice opposition doesn't mean he supports what you're saying. You can tell yourself a different story, but you're not necessarily in the right just because you haven't been confronted.
When you're on the opposite side – and you are seeking someone's understanding or that person's honest perspective – be sure you ask your questions in the right way, with the right context, under the right socially atmospheric conditions. If you don't set the stage for honest discussion, all the head nodding you see might just be people being polite.