Credibility of 'sold' prices on websites like Zillow, Redfin, etc

[Christopher Nguyen]

I've been trying to do deal analysis and finding comps that sold in similar areas- when I go on Redfin and flip the filter for 'sold' houses it shows the last listing price, but not the actual last sold price. On Zillow, it shows the last sold price and the source is 'provided by agent'. How reliable is the 'provided by agent' source? Is there a way to actually check to see if that is credible?