This comes from limited to no exposure to how far along you are now, but as long as you and the candidates are very clear about the roles and responsibilities needing to be filled right now vs. in the future, you should be in a good place to vet people further.
Folks who have "been there and done that" before will have a keen understanding of how, long before hiring a team, your new Chief Engineer may be asked to strap on headphones and code all day long to breathe life into your platform - which is not what a C-level engineer does by a long shot. So, I'd ask about their experience in facing responsibilities that are not typically C-level at all.
And, be sure you yourself know what your next 3-12 months needs are from these folks. An expert "roll up your sleeves" coder may be exactly what you need, but not all of them can ever become a C-level leader; which is fine and understandable.
Know what role you are filling, and make sure they know that too. Often, start ups I consult find that they just need someone to own and build the code, and that costs far less than a CTO, and can provide more realistic focus to the role at hand.