Watch out for people only living for themselves, not caring whatever they say as long as they get their way, trying to manipulate people intentionally. Look out for the ones that have no skill other than extortion, putting pressure. That always put work on others so they can never take blame. People who never apologize, and never admit they're wrong.
Those are the ones you'll want to avoid. Also always trust your gut.
Meanwhile he'll be playing the long con with you - manipulation and mind games that can be incredibly subtle. I had the misfortune of working with one of those for a few months - we almost became co-founders. Recognizing and getting out of that situation before I got in any deeper was one of the smartest moves I've ever made.
http://www.businessinsider.com/charles-schwab-ceo-takes-job-...
I'm not kidding, and this is a real issue. A lot of people who can be great entrepreneurs can also have detrimental effects on your culture, the trust of your clients, and your long-term sustainability. Drive, initiative, passion, aptitude - these are qualities present in great founders and appear to be present in workplace psychopaths.
Get to know this person, and get a strong sense of their values - not from what they say and want you to believe, but from taking in the context of their accounts of what has worked out, and what is not, in their personal and professional life. For people they speak dismissively or negatively of, find them and get their side of the story. You cannot do too much due diligence here - this is critical, especially if you start to become widely successful. Then you're locked in with your partner in the co-pilot seat, and you won't have the desire to bolt or ability to change your decision without significant if not disastrous consequences to your business.
Your values must align, and you must trust this person with your life. Your startup will be your life for the next 7-10 years, by your planning.