How someone treats anyone is how they'll eventually treat everyone; watch how they treat people who can't help them, and take notes.
The idea
How someone treats anyone is how they'll treat everyone. Eventually. They might be charming, supportive and friendly to you now, but if you get a glimpse of how they might treat you in a different phase (when you find yourself at odds with them, or hit a rough patch), take it as the gift it is.
How to do it
Watch how a potential connection treats the people who can't do anything for them.
If something feels off, trust that, even before you can explain it.
Success and a big network don't cancel out a red flag.
Take notes on what you saw.
Example
Ten years ago I had lunch with a more seasoned entrepreneur I hoped would mentor me. He was kind and interested in my work, until his soup arrived. He sent the bread back three times, with a master class in verbal abuse aimed at the server, and I couldn't see how to keep a relationship with someone who could reverse polarity that fast.
Watch out for
Maya Angelou: "When someone shows you who they are, believe them the first time." I'd add: and take really good notes.
Red Flag: They Only Talk About Themselves: Someone with zero curiosity about you in conversation is previewing the whole relationship; take note, and don't be that person yourself.
The Down Escalator: Every relationship rides a down escalator, so ending one just means stepping off; stop investing, decline politely, never ghost, and leave the door open.
The +2 Energy Rating: After interactions, rate how you feel from -2 to +2, rate three standouts each day, and give more of your time to the +2s.