How do I judge a conference's trustworthiness? I asked a question here How are travel expenses paid by conferences? , and the answer mentioned that it depends on how well I trust a conference (to pay back expenses).

That raises a new issue - how do I judge a conference's reputation (not in its field but in financial matters). Is there some objective way to judge that?
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