Well, he could if you wanted him to, but at this point I feel like it'd be sort of meta gaming to do so. Also, what does he have to lose? Amaretto has just proven himself to have the artefact of a God, shown that he's willing to steal from one, all to get back what he lost. That should convince most anyone that he's telling the truth is he dares to go to such lengths just to get the chance to see the again. And even then, if just one of the maids end up as being "mindless," that's all he needs. 

...Sorry if it seems like I'm trying to control the flow of events, but there is quite a lot of things to take into account here, so him just "suddenly" asking if Amaretto is telling the truth would seem a lot like "I don't want your character's plan to work, so I'm gonna ruin it with meta knowledge." 
Don't get me wrong, if Scalpel retains his immense paranoia and he does end up asking (provided all 3 are sane and have their memories), then I won't complain because it would make sense. I just really want to avoid meta-gaming to happen.