I think it is important to look for a person with a positive attitude. This can affect them in two ways (if not more) – they look for the good things that come out of difficult (bad?) things that happen and they are able to look for positive things in other people.
I also think it is important to know how a person plans to work out any issues that come up later on – is the person dead set against going to a therapist if the need really arises, do they have a Rav/RY to whom they go to ask advice and are do they know how to take the advice given?
Find a way to learn enough about the person to answer the issues you think are important, possibly including my two ideas above, and if you ask direct questions, try to understand what the person means by what they answered. Hopefully the person you are dating will have thought through some of these issues and have some answers to your questions, but you don’t want superficial responses, so you can use their response to dig a little deeper.