A good rule of thumb. If you tell your spouse, it’s friendly. If you don’t, it’s a date.
I’ve gotta say, that’s probably the best answer yet.
If the people want to sleep together, it’s a date.
I was going to say exactly this. Well not exactly. My wording was: "It’s not a date if you don’t want to bang each other. "
So if someone says When I get $, I’d like to take you for lunch, whats that? 
Do you want to bang each other?
I do but I was not the one asking, he was.
Is he asking you to lunch with the intent to bang you? I’ll answer that: yes. So then it is a date. Wear a low cut top.