The public should be on the lookout for symptoms of the disease.

公众应当留心这种疾病的症状。

Be on the lookout for someone whose color you admire.

注意你羡慕的某些人的发色。

Always be on the lookout for something positive to say.

时刻注意要说一些积极的方面。

This is something for which you must constantly be on the lookout.

这是你必须一直密切注意的事。

I'll be on the lookout for a new deck of CARDS for you if you want.

如果你需要的话，我可以帮你买一副新的纸牌。