You can refer to someone who sees or notices something as an observer .
例句
A casual observer would have taken them to be three men out for an evening stroll...乍一看的話，還以為他們三人是晚上出來散步的。Observers say the woman pulled a knife out of the bunch of flowers and stabbed him in the neck.目擊者說那名女子從花束中拔出一把刀來，扎進了他的頸部。
N-COUNT
觀察家;評論員
An observer is someone who studies current events and situations, especially in order to comment on them and predict what will happen next.
例句
Observers say the events of the weekend seem to have increased support for the opposition...觀察家說周末發生的事件似乎為反對派贏得了更多支持。Political observers believe that a new cabinet may be formed shortly.政治觀察家認為新內閣可能很快就會組建。
N-COUNT
（監督重大事件或局勢的）觀察員
An observer is a person who is sent to observe an important event or situation, especially in order to make sure it happens as it should, or so that they can tell other people about it.
例句
The president suggested that a UN observer should attend the conference.總統建議聯合國應派一名觀察員參加本次會議。
英漢詞典釋義
n.
觀察者, 觀察員; 遵守者;觀測者;目擊者
The old man was sent along as an observer.那個老人被派作觀察員。
英英詞典釋義
Noun
1. a person who becomes aware (of things or events) through the senses
2. an expert who observes and comments on something