For me the two most obvious ones are
1. When she lets people pass between us or walks on the opposite side of street lights and other obstacles when we are walking down the street.
2. She does not put her phone down when we are having a conversation.
For me these are a canary in the coalmine that she is not interested and I say a polite "Nice meeting you" and leave and go right back onto the dating app to see who else is interested.

What about you guys and girls?