Testing for Dengue Fever is a simple blood analysis which can be done at any hospital (and larger clinics). The result is immediate - once performed, so the only delay in getting an answer will be to do with how busy the hospital pathology lab is.
DF symptoms come on rapidly. You can have the test as soon as you (or your doctor) suspects DF.
Delay in diagnosing DF often comes about because many of the symptoms are mistaken, by the patient and the treating doctor, for influenza, kidney or liver disease, even meningitis and DF is not considered as a cause.
Remember DF is on the rise globally, and you are at risk in any tropical climate. If symptoms show up after you return home from a tropical climate, make sure you tell your doctor where you've been, because being unfamiliar with DF they may not consider it in their diagnosis unless alerted to the possibility.
Best treatment is prevention. See WorldNomads advice on avoiding vector borne disease here: http://journals.worldnomads.com/safetyhub/story/52684/Worldwide/Dengue-Fever-Fastest-Growing-Vector-Borne-Disease about 10 years ago
Some additional information for you on DF testing, and this is News released just this month (September 2010). There's now a simple test kit which can detect all 4 serotypes of DF in a single test (whereas 4 tests had to be done in the past).
The maker claim it reduces the overall testing for all types of DF from 1 day to 3.5 hours.
here's a news story about the kit: http://www.channelnewsasia.com/stories/singaporelocalnews/view/1081186/1/.html about 10 years ago