expression
please look; please see
Polite and honorific request to look at something. Used when addressing someone respectfully.
See also: 御覧なさる
この写真をご覧なさい。
Please look at this photo.
あちらをご覧なさい。
Please look over there.
expression
Used to emphasize that an earlier prediction or warning was correct. Often carries a tone of 'See? I told you so.'
ご覧なさい、やっぱり雨が降ってきたでしょう。
See? It started raining just as I said.
auxiliary verb
please try to do
Polite and honorific auxiliary verb attached to the -te form of a verb, meaning 'please try to do' or 'please do and see'. Usually written in kana.
この料理を食べてごらんなさい。
Please try this dish.
もう一度考えてごらんなさい。
Please try thinking about it once more.
ご覧になる is the standard honorific form of 見る (to see/look), while ご覧なさい is a polite imperative form used for requests or commands.
見てください is a polite request to look, but lacks the honorific nuance of ご覧なさい, which elevates the listener.
Derived from the honorific verb ご覧なさる (itself from ご覧 + なさる), with the imperative form ご覧なさい. The auxiliary verb sense developed from the imperative of ご覧なさる used after the -te form.