Is there a difference in meaning between "It's too late to telephone. Mary is asleep now." and "It's too late to telephone. Mary will be asleep now."？
Yes. The difference is in the reason given for it being too late to telephone. In the first sentence "Mary is asleep now", the speaker is stating（陈述） a fact. In the second sentence "Mary will be asleep now", the speaker is saying what Mary usually does. In fact, it's possible that Mary isn't asleep, but she is usually asleep at this time.
So let's take two situations. In the first case, imagine Mary's mother answers the telephone.
You: "May I speak to Mary, please?"
Mother: "No. It's too late. Mary is asleep now."
You: "Are you sure? It's only 10 0'clock."
Mother: "Yes. I went to her room and she was asleep."
The mother knows that Mary is asleep because she went into her room and saw her.
Here is another case. Two friends decide to telephone Mary.
Jack: "Let's telephone Mary. It's 10 0'clock."
Jill: "No. It's too late. She will be asleep now."
Jill knows that Mary always goes to bed before 10 o'clock, so now she'll probably be asleep. But it is also possible that she is staying up late.
In the first situation. Mary's mother _______.

A．is afraid to wake up Mary

B．wants to tell you that she knows Mary is asleep

C．knows that Mary wants to sleep at this time

D．wants to know if she can take a message for Mary

In the second situation, Jill _______.

A．knows that Mary has gone to bed

B．doesn't know whether Mary is asleep

C．is sure that Mary cannot answer the telephone

D．thinks it's possible that Mary cannot answer the telephone

In which situation do you think the word "will" is used in the same way Jill uses it?

A．Will you please lend me a hand?

B．It's too heavy, I think. I'll help you.

C．Don't get into his office. He will be angry now.

D．You won't have to come to my office. I will send it to you.