How do you define bright? Assuming bright to mean able to achieve post-grad honors:
I feel like you could easily distinguish if someone is bright or not within a few minutes of chatting.
I think that regardless of the subject of discussion, there are a number of strong indicators in the way a person speaks. One example is confidence: very rarely have I met a person I'd call intelligent that is also absurdly confident in their beliefs.
I realise my above statement is highly interpretable. What I mean is intelligent people seem to allow for their assumptions to be flawed, and be open to being proven wrong, about stuff that they can be proven wrong on (i.e. belief about a mathematical postulate versus belief in an objective truth such as global warming is real).
If someone is not bright, then when you start discussing a topic that is outside their everyday experience (i.e. not their job, not their hobby), they only have three options: (1) ignorance and indifference "huh?" "I don't know, this seems stupid/boring", (2) repeating a statement they heard from TV or radio, but unable to e.g. explain what that really means, what would be the consequences thereof, and (3) just saying something completely random.
A bright person would be able to admit their lack of knowledge in the topic, but then do some independent thinking, or ask additional questions, or... something.