how do you know someone is calling from a fake number?

I have my phone on two different ringtone settings...my contacts play music when they call and all others ring like a regular telephone. When it just rings if I don't recognize the number 9 out of ten times it's a scammer or "junk" call (sales pitch). Once in a great while it might be a wrong number, but that is rare. When I'm expecting a call I'll temporarily put that number into my contacts so that music will play when they call. That's how I can tell 99.9% of the time.