1 What are their priorities?
If they're all about getting drunk on cheap bottles of Chang, or bringing Swedish girls back to the beach shack, while you want to do yoga and learn to surf, it won't work.
2 What is their budget?
Nothing drives a rift between travellers faster than disagreeing on what's a sensible spend on a meal or hotel. Talk this through before you book any flights.
3 Are they good in a crisis?
You need someone who'll roll with the punches. Do they have a history of spinning out if they can't get a taxi home at night? Do they blame others when they're late? Do they hold grudges or sulk if things don't go their way? This behaviour is just about tolerable on a group night out, much less so when you're hot and bothered and trying to find a hostel for the night.
4 What do they contribute?
Are they too shy to approach a stranger and ask for directions? Will they make an effort to learn the language or will they expect you to do all the talking? Will they research and book accommodation along the way, or will you be thrust into the role of unpaid tour guide? It's fine for you both to have different roles and strengths, but a healthy balance of chores is key.
5 Do they make you laugh?
Will they help you see the funny side when you accidentally book a night in a brothel? Will they magnanimously laugh it off if you lose their jandals in the sea? Laughter is the most cleansing thing in the world, and we all need to travel with people who laugh easily, and coax laughter out of us.