can people see if you have ptsd or struggle with your self confidence

Sometimes 🤷🏻‍♀️ Best thing you can do is just keep working on grounding exercises and developing that self-confidence until it feels genuine to you. That said, everyone has lousy days sometimes. Nobody is a constant pillar of confidence and calm.

We all make quick judgements of our surroundings and sometimes people stand out. You should not assume that you stand out unless people treat you directly and pointedly as though you do. Not just passing glances and minor words.

The ones that notice anything outside of themselves? Because most people are more concerned with how they themselves are being seen than paying any attention to strangers unless they directly interact with you. So, Yes. But what they see may not be what you are feeling, but their own perceptions of what that means to them.
And some can tell. Some care, but can't do anything. you're a stranger. And some will see it as their way in to take advantage.

No.