look/looks

名词 look 作“看”“表情”解时, 是个可数名词, 有单复数形式:

1. She gave me a strange look. (LDCE) 她奇怪地看了我一眼。

2. I knew she didn't like it by the look on her face. (LDCE) 看她的面部表情我知道她不喜欢这件东西。

3. There were angry looks from the neighbours. 邻居们显出生气的样子。

当表示“外貌”“容貌”或“美貌”时，则应当用复数形式 looks:

1. His looks are really quite remarkable. (Wilde) 他的外貌确实很漂亮。

2. The twins were almost identical in looks. 这对双胞胎长得几乎完全一样。

3. She's beginning to lose her looks. 她在渐渐失去她的美貌。

4. I didn't marry him for his looks. (EED) 我并不是因他的美貌才嫁给他的。

5. She kept her looks even in old age. (LDCE) 她虽然老了,但美貌依旧。

因此,你的引文应改为:You can't judge a person by his looks.