BUILD TRUST.
I agree that it's hard to know. I just feel whether someone makes an effort to build trust one brick at a time. Starts by them being reliable, contributing enjoyment of sharing basic life things. Them acknowledging when I offer kindness.
HONESTY
I don't want to know a person's entire life history the first time we meet, but people who are honest when asked what they would prefer. People who ask for small things that they need them but who can respect a genuine "yes" or "no" in return.
GOODWILL
People who can say "I'm mad at you, but all in all you're a good 'un.
JOY
Purposely doing things that nurture joy in a friendship is important to me.