The rule is very simple but very hard :D
It's very simple because the rule is: if the first character of the verb can be used alone as a verb, then the verb can be separated. Otherwise, the verb can't be separated.
For example, 睡觉 is a verb, and 睡 can be used alone as a verb too. ---- 他正睡在我的床上。
见面 is a verb, and 见 can be used alone as a verb too. ---- 我想见你。
喜欢 is a verb, BUT 喜 can't be used alone as a verb. So 喜欢 can't be separated.
So how could we know which verbs's first character can be used alone as a verb?
Now this is the hard part. I use 睡觉 as an example:
睡 and 睡觉 are all verbs. Furthermore, they have very close relation: both of them mean: sleep.
But they are not the same thing.
睡觉 = 睡 + 觉 (verb + noun)
睡 is a v.t., BUT 睡觉 is a v.i. (I hope u know the difference between vt and vi. vt can be followed by the object, vi can't)
见面 = 见 + 面
见 is vt, 见面 is vi.
我见(到) something. 见 is vt, something is the object.
我和你见面。见面 is vi, there is NO object.
我睡了一个女人。 睡 is vt, 一个女人 is the object.
我在床上睡觉。 睡觉 is vi, there is NO object.
In fact, what I am saying is NOT exactly correct.
As u see, we must add “到” after “见” to make it as a vt.
Ur question is very good. Because it touched the essence of chinese language.
There is only one method to answer ur question: to speak chinese as a mother language..... lol...