When Kaede and Shuichi ask Lucas whether he's sure the other Relics were destroyed, he says yes without hesitation, although he does give them kind of a funny look at the question. After all, that's the whole point of this endeavor, right? Insight and intuition give both characters a bonus when it comes to sniffing out deception, but in this case, it's simply not needed. Lucas is telling the truth.

You don't think Lucas would lie to you, do you, Kaede?

The only thing they're really able to pick up other than the above is that he's really, really frazzled. He got yelled at by a mom earlier, and he's under a lot of stress. As a result, when Kaede reassures him that he's doing well, he gets kind of sniffly and flustered and will try to scuttle away to protect his pride if they don't have any other questions for him.