I'm a Westerner so I know that our politics are unpopular here. With this in mind I try to look for common ground on specific issues. Usually my red flags are when I detect a naive reactionary or anti-solidarity sentiment.
E.g. 1: one time I was on a second date with someone, and they started hating on homeless people towards the end. This led to me not dating them further.
E.g. 2: People who say racist things about South Asian people. This is very specific to my geographic context, but basically they're probably the most highly persecuted immigrant population within my slice of the world and face a fuckton of systemic racism. Obviously racism in general I find intolerable, it's just that due to South Asian immigrant vulnerability, this one stands out as very immediately and acutely harmful as well.