Valmiki Ramayana Sundara Kanda Sarga 15 Shloka 26

विशालाक्षीम् largeeyed lady, अधिकम् very much, मलिनाम् soiled, कृशाम् thin, ताम् her, समीक्ष्य after observing, उपपादिभिः by establishing, कारणैः with reasons, सीतेति She is Sita, तर्कयामास he started deliberating within himself.

After seeing the largeeyed lady worn out and dressed in soiled clothes, Hanuman started deliberating in himself and guessed it was Sita for strong reasons.