Fluidly using the right jargon, and signaling that you 'know stuff' without sounding like you're trying hard too show that you know stuff, requires a fair amount of intelligence. (Incidentally, an inability to maintain a natural flow of conversation when someone knows a lot of stuff is one way highly intelligent people reveal that their social acuity is not that high. Their IQ may be extremely high, but a five minute interview can often easily identify these things.)
A certain degree of being articulate and appropriately assertive can be trained – I think I see this happen in the military. However, I don't think it's a fake signal, I think this training really results in greater general intelligence, or greater ability to succeed in any case.
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.