I suppose that it is for me, as the party guilty of committing this ambiguity, to answer, not with an excuse, but with an explanation:
I have written elsewhere on ELU of the distinction between spoken and written English:
- Spoken English is governed by the Tolerance Maxim: Whatever should be understood may be omitted.
- Written English is governed by the Adamantine Law: Whatever can be misunderstood will be.
I might have expressed that more accurately as a distinction between composed and conversational or improvised English, for the ‘written’ style of English is often carried (with more or less competence) into ‘conversational’ situations, and the past hundred and twenty years or so have seen the ‘conversational’ style invade ‘written’ contexts to an extent the language has not seen since the middle of the 17th century.
My own style illustrates this point. In Answers I generally follow the Adamantine Law, adopting a strictly composed style and making every effort to anticipate and eliminate such ambiguities as that you detect in this sentence. In Comments, however, which I dash off with reckless indifference to academic niceties, my style is more conversational; I permit myself the slovenliness which the Tolerance Maxim indulges.
The sentence at issue derives from a Comment in which I put down the words more or less as they occurred to me. My inner editor automatically placed the relative clause which confused you immediately after the term it modified, words, for that is the position which a relative must occcupy. But I gave no thought to where in a dictionary should be placed. It never occurred to me that anyone encountering look up and in a dictionary in the same sentence might conceive them to be unconnected. Consequently, the connection was something which should have been understood; it might therefore, under the Tolerance Maxim, be omitted rather than made syntactically explicit.
But, alas! the medium was writing, taken to be composed, not conversation, understood to be improvised. The head modified by in a dictionary could be misunderstood; and according to the inexorable workings of the Adamantine Law, it was.
As for your question:
I regret to say that I know of no template or rule for placing adjuncts which might have forestalled this misunderstanding. There’s really no way to approach problems like this except to keep your ears open to what you are saying, and to be prepared to move your phrases and clauses around until the problems disappear.
And often even that won’t be enough. That method, for instance, might have led me to write this, which is certainly unambiguous:
Have you looked up in a dictionary the words which confuse you?
But that is a sentence of such exquisite godawfulness it makes my flesh creep. —What I needed to do was throw away my first formulation and find another collocation:
Have you consulted a dictionary for definitions of the words which confuse you?