Narrow the field. Make a list of critical needs―good schools and medical care, available property, easy access. Then, pursue additional research online.
Know the rules. Once you have a destination in mind, check immigration requirements closely. Most islands have strict controls over who can work, own property or businesses, and seek permanent residency or citizenship.
Live a little. Visit schools, banks, and hospitals. Talk to locals and expatriates. Experience life as it is for residents. If you start to feel isolated after a few weeks, island life may not be for you.
Read up. What is it like to live on an island? These books shed a little light: So, You Want to Live on an Island by Gay Morse, available at islandslife.com, and Which Way to the Islands? by H.G. Nowak, available at barefootman.com.
If you're in the market for an island move, check out So You Want to Live on ... an Island for things and places to consider.