Hang out with his family: Spending time with people close to him will give you an insight into the kind of live-in partner he would be.
Get him drunk: If he is the kind who comes home and pukes for three hours, you definitely need a rethink.
Get him out of his comfort zone: If he is the relaxed, order-at-home type, take him to a fancy restaurant; if he is an introvert, make plans to hang out with a huge group. Putting him out of his comfort zone will give you an idea of how he will deal with uncomfortable, unforeseen situations.
Make an elaborate weekend plan, promise to pay half and then don’t: We don’t mean for you to be evil, but this will give you a great understanding of how he will deal with financial crises once you both move in together.
Cancel on him and go out with friends instead: Moving in together and having all that quality time is great but you also need to have your own space. See how he reacts to you cancelling on a plan and opting to do your own thing instead. Of course don’t wait till the last possible minute to cancel; give him some advance warning but make it clear that you are cancelling because you have other plans.