He told her that I love you.

In 1, "I" refers to the speaker, while "you", the listener. In 2, they are "he" and "her" respectively.
Is this rule only applicable only in writing? In speech, how can you tell the two if "that" in 1 is omitted? Just by the tone or context or is it indiscernable?

1. He told her that I love you.
2. He said to her, "I love you."

not a teacher

In 1, "I" refers to the speaker, while "you", the listener. In 2, they are "he" and "her" respectively.
Is this rule only applicable only in writing? In speech, how can you tell the two if "that" in 1 is omitted? Just by the tone or context or is it indiscernable?

1. He told her that I love you.
2. He told her, "I love you."

#2 is not very natural.

He said to her, "I love you".
or
He told her (that) he loved her.