Valmiki Ramayana Yuddha Kanda Sarga 17 Shloka 61

राजन् O king अन्तस्स्वभावै: his intention, र्गीतै: tone of words स्तै: his, पश्यताम् to find out, भृशम् excessive, नैपुण्यं: dexterity, परस्य: other's, भावः feelings, सहसा: at once, बोद्धुम् to know, अशक्यम् not possible.

"O king It is not possible to know the intention (of an enemy) of others at once without excessive dexterity. It is to be known by his tone of words and description."