Unfortunately, I think most of us learn through trial and error! You have your "golden" families, your "nightmare" families, and most of them fit somewhere in between.
There is no hard and fast way to figure it out, as sometimes the people you thought were great end up being total idiots, and vice versa.

Some good signs:
- they are comfortable putting the child down and allowing him/her to play while you chat
- they ask thoughtful and relevant questions
- their location and schedule make sense with your own
- they are willing to work WITH you, as opposed to telling you "how it's going to be"
- they seem to be listening to you, and taking in the information

Some red flags:
- peppering you with odd questions
- inability to allow the child to get comfortable in your home (although keep in mind that some parents use their kids as safety blankets when in an unfamiliar setting)
- trying to dicate your policies ("Well, it says naptime is at one, but Jr doesn't sleep until 2:30, so you will have to change that") or rules
- asking for special favors or requests (that seem excessive) ("Can you please watch him for an hour earlier and an hour later, but I can't really pay you extra?")

And I absolutely watch how parent and child interact. I always try to meet with both mom and dad, so that we all have a chance to get to know each other. If I feel immediately uncomfortable with the family, and continue to feel that way the entire interview, chances are that we are not a good match! It's totally normal for everyone to be a little nervous and edgy, but I have learned to listen to my gut.I am about to be a pretty huge part of these peoples' lives, and they mine. We have to work togther to raise these kids, and I want to work with people that I can more than just tolerate!
Best of luck filling up your spaces.