I suppose you are talking about a situation where you bumped into a stranger and he said something sarcastically. I don't think in that case native speakers would say anything similar to "you're very observant". Instead, the following are some common responses: 1. 你盲㗎? (nei5 maang4 gaa2) Are you blind? 2. 你行路唔帶眼㗎? (nei5 hang4 lou6 m4 daai3 ngaan5 gaa2) Don't you "bring along" your eyes when you walk? As you can imagine, these are offensive. The following is less offensive: 3. 小心啲吖嘛! (siu2 sam1 di1 aa1 maa3) Be careful, won't you?

If you want to carry the same meaning as in English, you can say in a sarcastic way: 好眼力喎！ <-- remember to add a "!" in writing, or pronounce "好" in a stronger tone in speaking. 好 = good 眼力 = visual capability 喎 <-- ending particle, adding emphasis, and make the tone smooth. The subject can be omitted. This phrase is informal, sarcastic, but not offensive. But in Chinese culture, if we "walk past" someone, we usually won't say in that way. Saying in a sarcastic way in such situation will make people unhappy or make them feel guilty for being "perceptive". Then you will have no friend. Just do it more directly and say hi to them. 嗨 = hi hi, good morning. hi, good afternoon. if we "run into" someone, then saying in a sarcastic way is reasonable.

We normally say '你觀察力好強. It's similar meaning, because we don't say 你很敏銳或洞察力很好。 In informal way. Hope it helps.