Yes, we can get at least some idea of a person's intelligence based on what they post here, but, obviously it's just going to be an estimation. Sometimes, it might even be completely off.
On top of that, our estimation of another person's level of intelligence is also subjective to a great extent because it will vary depending on our own intelligence, our perceptions, and our beliefs. Even outside the internet, our estimation of the intelligence of other people that we meet and know is always going to be subjective.
What people simply write on an internet forum only demonstrate a very, very small portion of their intelligence.