I've found the best way to know if I'm going to work well with someone is to have previously worked with them before. Look for people you've enjoyed working with, be it a co-worker, classmate, someone you've worked on open source with.

Things that I think must exist between cofounders.

* You must like and trust each other.

* Make sure you are able to be candid with each other.

* Know each others weaknesses. Really another way of saying have realistic expectations.

If you have no prior work experience try working on a small one-off project to get a feel for each other's working styles.

It's a tough one but you don't really know until you start working together.

One thing that's been important is communication and openness. Trust that you're going to be upfront with each other. Otherwise, the issues can't come out and be dealt with. Instead, they get buried away until one day it blows up in your face.

Only a few weeks ago I bundled up some of my experiences - as cofounder, or as an advisor - into a post on the key questions I see [1].

The big one that gets overlooked is the Behavioural preferences - appetite for Risk, are you a Manager or a Maker, etc. The commercial opportunity and personal passions may align, but when something goes wrong are you going to respond in similar ways or will your cofounder start making decisions 'on their own' [2]?

[1] http://jacobaldridge.com/business/8-key-questions-to-ask-whe...

[2] NB They will probably feel you've gone off on your own as well. It's just because you've responded differently - for example, doubling down on a risky decisions versus backing away from it.

Same as the previous answers - by having worked together before and building mutual trust.

For people that didn't have that advantage, maybe something like a hackathon can be a good litmus test?