How do you tell if a surname is compound or not?

The answer to this question  is rule 22.5C6, "Nature of  surname uncertain."  It says, "If a name has the appearance of a compound surname but its nature is not certain, treat it as a compound surname unless the language of the person is English, Danish, Faroese, Norwegian, or Swedish."

If the language of the person is English, enter under the last part of the  name and do not refer from the preceding part, per rule 22.5C6.

If the language of the person is Hebrew, treat the surname as compound if its nature is not certain.
Fortunately or unfortunately, "certainty" is in the eye of the beholder.

Israeli names like this are often hyphenated in one source and unhyphenated in another.  If a hyphenated form turns up, that provides "certainty" that the  surname is a compound one.  So stay aware of this kind of thing, and record all variants!