Both, depending on the circumstances. 

If the families were well-known to each other and their interests likely to align – the most common scenario we’re talking about is neighbors looking to consolidate their estates – you’d probably be pretty frank about it.

If on the other hand, the two families are not well-known to one another, they’d probably get more subtle about it: have the two sides meet at social occasions, see how the young people in question get along (this is why formal balls and dances were created, and why it’s very strange that cotillion culture still exists), then bring it up in such a way that neither side could lose face, probably as a hypothetical or something.

On the other hand, we have plenty of examples in Westeros of people being very blunt about offers out of the blue and rejected offers leading to hurt feelings, so…