Does this stage inevitably turn engineering into a feature factory just to close deals? I'm worried the roadmap will become 'whatever the client screams for' rather than building a scalable core. Any specific questions I can ask the founders to sniff that out?
It might be better to dance around such questions. Don't directly challenge their product strategy. Instead, be a little sycophantic, encourage them to talk more, tell you more. Get excited by the features they tell you about, ask questions about how they landed on that feature set, etc. But then pay attention to what they don't say. Pay attention to if they say "we had an idea" vs. "our customers helped us refine" vs. "our founder has a vision". Listen for gaps in their stories, hesitations, anywhere that they get a little nervous. Pauses and rephrasings as they tell their story will also tell you their problem areas.
Basically, just keep them talking. Read between the lines. Teams at this stage tend to be really bad at hiding the warts in their organization, if you just give them enough space to let it all out.