How do I distinguish between a purpose clause and a result clause?

Students often have trouble distinguishing between a purpose clause and a result clause when they meet them in the Latin. This is because they look quite similar, as they are both formed, when positive, by ut + imperfect subjunctive.
Purpose: domo excessi ut Romam nocte advenirem. - I left my house so that I would arrive in Rome at night.Result clause: domo excessi tam celeriter ut Romam nocte advenirem. - I left my house so quickly that I arrived in Rome at night.
These two sentences are very similar, except the Result Clause contains the words 'tam celeriter (so quickly).' Most result clauses have sign post words like 'tam (so)' that distinguish them from Purpose Clauses; other sign post words include tot (so many), ita (so), and sic (so). If you see one of these words, then you are most likely dealing with a result clause. You can also easily tell the difference between the two clauses when they are negative, as negative purpose clauses are formed by ne + imperfect subjunctive, and negative result clauses are formed by ut non + imperfect subjunctive.