Going through my list of questions, some will draw out motivation more than others if you listen well.
What is your remaining mortgage balance? I've found that typically unmotivated people will ask you why you need this, the motivated ones would give you their account number for the asking.
What is your mortgage payment? Same as above.
Are the payments current on the house? Good sign here of motivation, although a lot of people never get motivated enough to do something about it
Why are you looking to sell this house? I have found unmotivated folks to say: I'm looking to buy a new house in a different part of town, I just wanted to see what you would offer, I'm tired of this house, I've had the house listed for a couple weeks and no offers at asking price.
What is your asking price? Having them tell you "what I owe" isn't always a good sign of motivation but is more so than not.
What is the estimated value of the house? Ask this right after "what is your asking price?" and it will be a huge help in determining motivation. I think you'd be surprised at the amount of people who knowingly ask for more than its worth... BY THEIR OWN ADMISSION!!!!
When do you want to move? The asap answers are usually good motivation. This summer, or when I get a good price, or whenever it sells typically aren't good answers for me.
Is your house listed with a real estate agent? The yes or no usually doesn't help, but the explanation that comes along with the answer is good.
If you were to list the house with an agent, what repairs do you think would be necessary to put the house in retail condition? and pair this with How much do you estimate those repairs would cost? Many people think that new carpet, paint, and updating bathrooms is only $2,000 for a 2,000 sq. ft. house. Take their estimates with a grain of salt, again, the explanation is more indicative than the number itself.
Would you be willing to allow us to take over the payments on your house? This question is obviously eluding to a subject-to deal but will help determine motivation nonetheless. Not quite 100% but darn close of the motivated folks will say yes.
When can we arrange a time for me to see the house in person? If you get a "my grandkids are in town for 2 more weeks, after that would be ok" it's not a good answer. The "I'll be home from work in 20 minutes" really could go either way on motivation.
Hope this helps, good luck to you.