“We should get coffee sometime.”
You’ve had a good conversation. Someone says this as you’re leaving. It sounds like an invitation, but there’s no day, no place, and nothing to put on your calendar yet.
That doesn’t make it insincere. The person may want to see you again without knowing when they’re free. They may also be expressing friendly interest that hasn’t become a firm intention. The sentence alone won’t tell you which.
If you’d like to meet, you can give the idea somewhere to go:
“I’d like that. Are you around Saturday morning?”
Or keep a little more flexibility:
“Definitely. What does next week look like for you?”
Notice how small the move is. You’re offering a next step, and leaving room for the other person’s schedule.
“Saturday works. How about ten?” gets you closer. “I can’t Saturday, but Sunday’s good” does too. Both responses help build a plan. If the answer stays vague, you can leave it open: “Sure—let me know when you’re free.”
And when you’re the one suggesting coffee, a little detail can make your interest easier to respond to: “We should get coffee. Are you free one morning next week?”
You don’t need a special rule about what Americans really mean. Pay attention to the conversation you’re having, and whether you’re both helping the plan take shape.