I think any time is fine to ask "Are you OK?" In Chinese, 你好嗎？is as common as hello. The reply indicates the level of trust they have in you. If they say "Yes

I think any time is fine to ask "Are you OK?" In Chinese, 你好嗎？is as common as hello. The reply indicates the level of trust they have in you. If they say "Yes, of course!" then there's nothing to worry out. "Fine" means they're not fine, but they don't trust you enough to tell you why. If they actually tell you what's really on their heart, you now know that this person thinks of you as a trustworthy friend!