Sound Loans in Chinese
Let me try to answer your question, though I am not a professional in Chinese language.

Seems you are talking about the so-called radical-phonetic characters, and you are asking a way to spot them among tons of Chinese characters. I have a trick here for you, since I always use it when I, as a Chinese, don't know the character.

First of all, you need to be able to know which one is the radical part, normally either on the left side or on the top, e.g. 沐. The radical part here is the "three drops of water". So that you know, aha this has something to do with water.

Then, the most important, you must be able to pronounce the phonetic part. E.g. 沐's phonetic part is 木, and it is pronounced as "MU".

At last, the rule of thumb is, you just pronounce using the phonetic part! No matter what. 90% you will pronounce it right (maybe the tones need some adjust). If you are wrong, there will be people correcting you, and then you remember next time. :D

Simple, right?

Examples are as follows: 尧 is pronounced [yao 2]. It is the name of an ancient legendary king. But when you see 烧[shao 1], 浇[jiao 1], 饶[rao 2], they don't pronounce as [yao 2], BUT you may discover that they share the common vow. Another example is: 提[ti 2], the right part is 是[shi 4], which has totally a different sound.