I’ve known my co-founders for 8 months, 10 years, and 12 years. The one I knew the least amount of time, was the one I could trust the most.
Look for flags. If they do anything at all untrustworthy — they will be untrustworthy. If they play games with equity. If they don’t contribute equally in any way. If they hedge. If they aren’t as committed as you. If they don’t believe as much as you. If they hide — anything.