Ask to talk with their previous co-founder

I am always looped into conversations that are asking about 'what you wish you've asked your co-founder before you started', and the most simple answer is – ask to talk with their previous co-founders.

Whether they will let you talk with them (or have an excuse not to), what they will say, etc. is going to be a huge telling card about what you are getting into. Don't skip this step.