I am starting the process of adding a technical Co-founder, I plan on splitting everything 50/50. I wanted to know from the community, some of the things I might need to be asking that could test knowledge and if there are any "typical" things that people say that would be considered a black flag of a potential candidate.
In a startup, one of the most important things is to be passionate about what you are working on.
The hours are long and the pay is not mutch, and under these conditions, passion goes a long way, so you are talking to people try to figure out their level of passion above all else, even skills because a passionate co-founder with moderate skill will be better for you that a high-level professional who does not care about the problem you are trying to solve.
And keep in mind that it is almost impossible to know if someone will be a great fit for your startup, so don't just add him/her with all privileges and 50% from day 1, but set up a vesting period where they can show their commitment before getting that shares.
I agree with @Adil that you should use vesting to slowly release equity based on milestones or time, that way you don't give away 50% to someone who walks away without doing anything.
The question is do you want to give away half of your potential business? A figure for a technical co-founder would 20-40% depending on how much work is invested; but 50-50 also works, but under the right conditions and shared goals and values.
There's no one size fits all question when looking for 'red flags', sometimes it could be an overinflated ego, sometimes it's their written experience doesn't match up with what they say.
A good non-technical question I heard today that I intend to use myself is "what's your vision of the company; long term growth or until fast growth then exit". Either could work depending on how you view the question, however if your view is long-term but your partner is fast exit then you're unlikely to be compatible, as your growth strategies won't be in-sync.
50/50 is a bad idea. Try and work with this person for 3-6 months to see if there is talent-culture-fit & Founder-Passion-Fit. CTO usually is 20% if they come on early as co-founder and stay till their options vest. Remember, there will be other roles to be filled once the product is ready.
Usually, there is a cliff of 1 year with a vesting schedule.
Don't do 50/50 at any cost. Your partnership will end, it's just a matter of time, so be very careful with % you are giving away. In the beginning, when the value is 0 it is really easy to give away a large percentage, but what if your business is $1B in 5-7 years, is it worth giving away $500MM? Also, there are some legal issues that may relate to, based on your local laws.
Also, your co-founder should complement your skills. What to look for? Experience and specialization but I think most important is to believe in what business will do, mission and goals, desire to succeed, positive energy. You'll be spending with that person 25 hours a day, so you should like each other.
The most important and key thing you should look for is the mindset of your Co-founder, he/she should not join you just because he/she wants to become a boss or start a company but someone should join because he/she truly believe in your idea and wants to solve the problem for the community.
50/50 is not a good idea. If you are too concern, you can start by getting a co-founder on a contract of 3-6 months and evaluate if he/she really fits or not.
Openness to an equity stake is key, you need total buy-in.
I would definitely get a Non-disclosure signed from the prospective Co-Founder. I did not and the electronic engineer who contacted me through Co-Founders Lab presented himself as an independent engineer to develop my prototype but was in fact working for Ring Video at that time. Unfortunately the engineer (Craig Anders) and Ring Video stole my patented idea. Don't let this happen to you.