How do I judge a conference's trustworthiness?
Financial reliability is going to be related to scientific validity. Definitely agree with JonCuster's comment that long-standing conferences (e.g., annually recurring events with a history of substantive organizers and programs) are more trustworthy. Some new conferences are hard to evaluate; this will depend more on who exactly is organizing and whether your network trusts them. Otherwise, this general list could help.

https://libguides.ucd.ie/publishing/predatory


  
  The conference has an overly ambitious title
  The technical programme is broad. Very broad.
  The language on the conference website is…off
  Renowned organisations are sponsoring a low-profile conference
  The organisers’ contact details are missing, or aren’t quite right
  Another conference with a suspiciously similar name already exists
  The conference or its organisers have known associates  
  The organisers are charging higher-than-normal fees
  The conference is unusually frequent
  


Caltech Library’s list of predatory conferences: 


  https://libguides.caltech.edu/c.php?g=512665&amp;p=3503029