you are much more observant than I gave you credit for.
我没想到你有这么敏锐的观察力.
或：你的观察力比我想象的要高得多.
give someone credit for something:
认为某人能够做什么事,有什么样的能力.不能直接翻译成赞扬什么.
例如：I did not give you credit for such skill.我没想到你有这个本事).