(Please) look — a polite/respectful imperative used to ask someone to look at something.
ご覧なさい、この写真を。去年の旅行のものです。
Please take a look at this photo. It's from last year's trip.
先生：ご覧なさい、ここに重要なポイントがあります。
Teacher: Please look — here's an important point.
どうぞご覧なさい。ご不明な点があれば聞いてください。
Please have a look. If you have any questions, please ask.
Used to emphasize that a previous prediction or remark was correct — like 'See?' or 'There you go.'
ご覧なさい、言った通りに雪が降り始めましたよ。
See? Just as I said, it's started to snow.
Used as an auxiliary after the -te form of a verb to politely tell someone to try doing something (please try ~).
このボタンを押してご覧なさい。それで電源が入ります。
Try pressing this button. That will turn the power on.
ゆっくり話してご覧なさい。相手はあなたの言うことを理解するかもしれません。
Try speaking slowly. The other person might understand what you say.