It's gotta be a combination of the two. When you meet someone, you can't start off by doubting everything they say, and assuming they're lying. It's not practical, will negatively influence how you treat people, and makes you not a fun person to be around. So a certain level of trust should be given until they prove they can't attain it.
However, that level of trust only goes so far. You can take them at their word up until the point that you risk something. So you can take them at their word immediately, but you can't depend on their word immediately. As examples:
If they say their IQ is 150 - trust, They say they'll use their brainpower to write your research paper for you - don't trust.
They ask to borrow a dollar until tomorrow - trust, They ask to borrow $20 till next week - don't trust.
They say they find you attractive - trust, They say they are in love with you and want your children - don't trust.
Real trust only comes with time, shared experiences, someone proving they can be trusted with smaller things and working their way up to larger things. However, a parallel of that is the only way to find if someone is trustworthy is to give them a small amount of trust and see how they use it.