Is it possible to cast detect thoughts on a corpse that has had Speak with Dead cast on it, and would it be effective?
Would I be able to detect the surface thoughts of the corpse being questioned?
Yes? No? and why?
I'd probably go with no.
My reasoning is that Speak With Dead only "grant the semblance of life to a corpse, allowing it to answer questions".
Detect Thoughts affect "conscious creatures with Intelligence scores of 1 or higher". IMO a corpse with the semblance of life doesn't fall into this category.
But the subject pf the speak with Dead can actively try to oppose you. It's not like Speak with Dead turns the corpse into an answer dispensing machine. It still gets a will save so it still has some say in the matter on some level. It's this that I think you can make a case for "there's a mind at work still", at least during the duration of Speak with Dead.