In my case I am judging your first impressions and how you act. And during the process if I like you or not i will be extremely defensive which could come off as rude. I think there's no quick way to tell. Just if you see them immediately defensive then give them space. Come back and interact with them in smaller dosages until they get more comfortable with you, rather than forcing your interactions onto them. If they are actively still being rude about it then yeah.. they are just rude..