What is the best way to determine the success or failure of a welfare program?

The simple answer is no. I guess you could have data suggesting that a higher percentage of people are getting off welfare, or they are getting off quicker. But this also need a stable population to take your data from. If you really studied that data, you probably could come up with a reasonable idea of the successes and failures. I'm sure most politicians don't do this though. They probably get a small, insignificant statistic, and use it to show success.