Even you misunderstood that question. It's not "how do I trust this person key", but - "how much do I trust this person to sign other people's keys ". As in "

Even you misunderstood that question. It's not "how do I trust this person key", but - "how much do I trust this person to sign other people's keys ". As in "how much do I think that other person is careful in checking other people's keys". Not judging his key, or your ability to check keys. You are judging the other person capability to check another, third person keys. This is important for PGP "web of trust".