Clearly you will want to validate their skill set - this may mean acting like an employer about to recruit a new employee before offering to partner. You might

Clearly you will want to validate their skill set - this may mean acting like an employer about to recruit a new employee before offering to partner. You might also need to check that their history will not be a problem when you come to raise finance. Are they an undischarged bankrupt for instance (in the UK - are they disqualified from acting as a company director). Then it is all down to - can you work with this person? Plus, is this person likely to represent the business well in their selected role?