What I've heard is the dengue carrying mozzies are a smaller species that are active during the day, not at night, I believe any insect repellant will assist in keeping them away from you. A good friend of mine got dengue at Koh Samui a couple of years ago, he was hospitalized and said he was feverish and had severe body aches and pains. It lasted for about a week. Dengue has been fatal, usually with folk who are old and maybe have existing health troubles, also young kiddies are at extra risk. From what I've been told Koh Samui, Koh Phangan, and Koh Chang are very high risk areas. At Koh Samui there are large signs all around the island warning locals of the dangers of dengue. However, all these signs are in Thai, nothing in English as the authorities don't want to alarm tourists. This is a fact because I can read Thai and I lived on Samui a couple of years back.
You would know if you got dengue because the typical symptoms are fever and aches & pains. If you experienced such symptoms you should seek medical advice immediately.