I think each satellite data has to went through validation. Normally, validation is done through in situ measurements. I was one of the participant to collect soil sample for gravimetric soil moisture measurements, in situ SM measurements using theta probe. At the end satellite SM was validated using theta probe, gravimetric and airborne sensor measurements. Based on the daily measurements for about a month, I think conclusion had been made on accuracy. Hope it helps.
Have you had chance to look at SMAP handbook? Please go through page from 48 to 50, section II. Requirements and validation Metrics to get your answer in depth. Please let me know if you want me to attach the handbook.