In the word-group a tall boy each of the constituents is independently open to grammatical changes peculiar to its own category as a part of speech: They were the tallest boys in their form.
Between the constituent parts of the word-group other words can be inserted: a tall handsome boy.
The compound tallboy — and, in actual fact, any other compound — is not subject to such changes. The first component is grammatically invariable; the plural form ending is added to the whole unit: tallboys. No word can be inserted between the components, even with the compounds which have a traditional separate graphic form.
All this leads us to the conclusion that, in most cases, only several criteria (semantic, morphological, syntactic, phonetic, graphic) can convincingly classify a lexical unit as either a compound word or a word group.
Consider the following examples.
"... The Great Glass Elevator is shockproof, waterproof, bombproof, bulletproof, and Knidproof1 ..." (From Charlie and the Great Glass Elevator by R. Dahl)
Lady Malvern tried to freeze him with a look, but you can't do that sort of thing to Jeeves. He is look-proof.
(From Carry on, Jeeves by P. G. Wodehouse)
Better sorts of lip-stick are frequently described in advertisements as kissproof. Some building materials may be advertised as fireproof. Certain technical devices are foolproof meaning that they are safe even in a fool's hands.
1 Knids — fantastic monsters supposed to inhabit the Cosmos and invented by the author of this book for children.