In my opinion any technical co-founder material should own/operate/maintain
their own servers on which they host their own site and content, be an
expert in one or more languages, and own the implementation of technologies
all the way from the back end to the front end, while being able to
integrate 3rd party APIs in a stable way.
I think a fair test of this would be to request them to create a basic web
application allowing secure authentication and a demonstration of using a
3rd party API with local persistence. Such as a sample check-out workflow
using Stripe (free to sign up and create this test with).
Require that they
are also required to provide traffic and click analytics. Seeing how they
solve this will give you your answer if they are qualified.
This would ensure they are able to maintain their webserver to put up this
This would ensure they are able to create a user store.
This would ensure they can maintain rational persistence.
This would ensure they can consume a 3rd party API and integrate its
results with their own system.
Stripe is especially useful for this since
you could create the account, give them just the Test API keys, and watch
the results in the Stripe dashboard.
without core competencies outlined above, that stuff is useless. If they
have core competencies, then those abilities are icing on the cake.
And knowledge of how to do all of this will be essential to someone leading a team of people needing to do the same.