What are some subtle signs that you're being low balled on an offer? How do you counter it?

Why do you need subtle signs for something so specific? It's not a notion, it's just math. You set yourself a range that you'll be happy with (after you're familiar with what's on the market for your tech stack and experience) and go from there.

If you're just beginning, IMO it's fine to get just any offer and start (because let's be realistic, for your first job you won't have 10 different offers). If you're experienced, you should know your worth and you probably will have some more choice.

The interviewer grinning when they say the low ball offer?