In the Ramayana of Valmiki, Hanuman goes to Lanka and sees the body of Ravan's wife Mandodari, Hanuman believes that she is Seetha (even though she's not in actuality), so why did Hanuman not come in-front of Mandodari and talk to her during this incident in the first place? Sundara kanda chapter 10 The next chapter says that he realized that it was not Seetha, but how did he realize this?
It is mentioned in Valmiki Ramayana, Sundara Kanda, Sarga 10 that Lord Hanumana seen many women there in the palace of Ravana but when he saw Mandodari, because of her beauty and appearance he thought that she could be mother Sita.
taasaam eka anta vinyaste shayaanaam shayane shubhe |
dadarsha ruupa sampannaam aparaam sa kapiH striyam || 5-10-50
That Hanuma saw among those women a very beautiful woman sleeping on an auspicious couch arranged alone at a side.
muktaa maNi samaayuktair bhuuSaNaiH suvibhuuSitaam |
vibhuuSayantiim iva ca sva shriyaa bhavana uttamam || 5-10-51
gauriim kanaka varNa aabhaam iSTaam antaH pura iishvariim |
kapir manda udariim tatra shayaanaam caaru ruupiNiim || 5-10-52
Hanuma saw Mandodari with a beautiful form together with diamonds and pearls, well decorated by jewellery and with her self radiance as though decorating that great building with a fair complexion and with a radiance like golden colour, who was dear to her husband the lady of women in that gynaeceum sleeping there.
sa taam dR^iSTvaa mahaa baahur bhuuSitaam maaruta aatmajaH |
tarkayaam aasa siitaa iti ruupa yauvana sampadaa || 5-10-53
harSeNa mahataa yukto nananda hari yuuthapaH |
That Hanuma with great arms seeing the woman well decorated, out of logic thought thus: "This is Seetha by the wealth of her appearance and her youth". That warrior of Vanaras was delighted together with great pleasure.
aashpoTayaam aasa cucumba puccham | nananda cikriiDa jagau jagaama|
stambhaan arohan nipapaata bhuumau |
nidarshayan svaam prakrtim kapiinaam || 5-10-54
He clasped his arms, kissed his tail he was delighted, he was playful, sang, paced showing his simian nature, climbed pillars and fell down on land.
But before he think of talking to that lady an other thought came in his mind that mother Sita cannot sleep with peace without lord Rama. So, he concluded that that lady is not mother Sita. It is mentioned in Valmiki Ramayana, Sundara Kanda, Sarga 11.
avadhuuya ca taam buddhim babhuuva avasthitaH tadaa |
jagaama ca aparaam cintaam siitaam prati mahaa kapiH || 5-11-1
The great Hanuma then removing that thought became with the right mind and had another thought about Seetha.
na raameNa viyuktaa saa svaptum arhati bhaaminii |
na bhoktum na api alamkartum na paanam upasevitum || 5-11-2
na anyam naram upasthaatum suraaNaam api ca iishvaram |
na hi raama samaH kashcid vidyate tridasheSv api || 5-11-3
anyaa iyam iti nishcitya paana bhuumau cacaara saH |
That Seetha is not suitable to sleep separated from Rama, will not eat, will not decorate also, not suited to drink a beverage, to reach another man even though if he were Indra because there is indeed no one equaling Rama even among gods. This is another woman - thus deciding that Hanuma paced in that banqueting hall.
Here are my 2 cents:
When Hanuman saw mandodari, he decided that she is not seeta by observing her expressions while sleeping. She appeared calm and hanuman thought, any lady living with daemons away from her husband/family cant be so calm.
Note: I dont exactly remember where I read this so cant provide links to document.