How do you know your boyfriend has moved on?
When you no longer see him.
When he is just polite, and not emotionally invested anymore. Like you're just aquaintances.
That little move you give him no longer works.
He won't call, he'll never come see you, when you pass him on the street he won't even look at you and if you talk to him he'll just ignore.
How about when he stops talking to you.
I think the easiest way to find out would be to ask in a nice way and tone.
When he is very vague with you and no longer talks to you that much. Short answers over text, and over phone. These are some signs he is moving on, and since it is long distance he could possibly be cheating on you.
When he stops calling or texting and when he gets starts seeing someone else exclusively are two big signs...