Everyone does it informally to varying degrees for varying problems (often times subconsciously). E.g. "What do I know about this person? Can I trust them around my kids?"
You verify trustworthiness by research. Who is involved? Do I trust anyone who trusts them? What are their motivations? What would cause them to take action against me? What causes them to protect my interests? What laws are they subjected to (i.e. who can coerce them)? What do they say for themselves? Where do their words fall on the credibility to BS scale? What is their reputation in the community? What do their competitors/adversaries say? What would cause their behavior to change?
I won't enumerate all my research on Mullvad. I can say Mozilla attaching their brand to Mullvad's services helped me a lot (trust by proxy). I'll also say that some of their product decisions give credibility to their anonymity claims. Lastly, I found someone who shared a competitive analysis across many providers. I found the analysis trustworthy. Mullvad has some weak points, but was still the best provider for my particular use case.
You verify trustworthiness by research. Who is involved? Do I trust anyone who trusts them? What are their motivations? What would cause them to take action against me? What causes them to protect my interests? What laws are they subjected to (i.e. who can coerce them)? What do they say for themselves? Where do their words fall on the credibility to BS scale? What is their reputation in the community? What do their competitors/adversaries say? What would cause their behavior to change?
I won't enumerate all my research on Mullvad. I can say Mozilla attaching their brand to Mullvad's services helped me a lot (trust by proxy). I'll also say that some of their product decisions give credibility to their anonymity claims. Lastly, I found someone who shared a competitive analysis across many providers. I found the analysis trustworthy. Mullvad has some weak points, but was still the best provider for my particular use case.