可以把 only 放在 by 的前面。例如：

We can detect the presence of an electric current only by the effects in produces. （only 把词义集中在介词短语上）只有通过电流产生的效应才能发现电流的存在。（摘自赵振才：《英语常见问题解答大词典》增订版第五版p.1046）

【1】Admission to the school is by examination only.

备注：only为副词放在名词后面，修饰名词.only相当于alone，examination是介词by后面唯一的宾语(内容）即介词by后面的宾语不能其它宾语(内容）。

【2】Admission to the school is only by examination.

备注：only为副词放在介词前面，修饰介词短语.仅仅只有通过考试，介词短语是唯一的方式，不能是其它方式。

总结：only当作副词，可以放在名词后和介词前，这是侧重点不同。强调内容不同。

The bar is for members only. 这间酒吧只对会员开放。

备注：only修饰前面名词members. 强调只能是会员，其它人不开放。

He was identified only by his uniform and personal belongings. 他的身份只是通过制服和个人财物确定了下来。

备注：only 修饰后面介词短语，强调这是唯一的方式，其它方式不可以或者不能确定他的身份。

Admission to the school is by examination only.

这个句子是牛津高阶第三版修订版1981上的，在admission词条下。把only放在by前面当然是可以的。如果硬要按照by examination的搭配来查找，only在前在后例句都少，两种用法都有在用，但是句法不同，举几个：

1.Officially, enrolment was only by examination, with 2000 children competing for 150 places, but politicians’ nominations would take up at least 25 per cent of them.(Delhi : adventures in a megacity一书第192页)

2.A voyage has the same meaning but is only by sea.(朗文当代英语大辞典英英英汉双解缩印本第1873页travel词条下)

3.These methods of determining the pricing structure are almost unheard of in other types of businesses. If they served photographers well, it is only by accident, not by purpose or design.（ASMP professional business practices in photography第167页）

这个句子“This is only by way of analogy.”确实是汉英词典上的，在“比”这个汉字条目中。