I remember the pre-Internet days when your only source of reliable information was a library or friend. The Internet has radically shifted many of our life processes and travel is one of them.
Travel review sites are a fantastic way to evaluate hotels, resorts, airlines, attractions and almost every other aspect of travel. 

The one site I always check (the 800lb gorilla) is TripAdvisor (link). With millions of reviews, you are almost guaranteed to find what you are looking for. There are of course many others like CitySearch (link)

Don't forget the traditional review sites like Yelp, OpenTable, etc. Using these types of sites we have almost always gone to great vacation resorts. Before booking, we know the cleanliness of the resort, quality of restaurants, level of service, etc. 

As you read these sites, it is important to keep certain elements in mind:

• think about the mindset of the reviewer. One resort we visited had very positive reviews except one of them gave it a 1/5 stars. When we read the review, we realized the reviewer was a baboon because he gave the resort a bad review because one of the restaurants had dared to give him non matching cutlery. Let's just say that's not something that would bother me.
• give more importance to recent reviews. We were evaluating a resort in Cuba which had a fantastic overall rating but as we read the more recent reviews, we realized the resort had started to be neglected (but the reviews dating 6 months or more were glowing).
• search for pictures. I always look for pictures and generally do a Google Image search to find pictures of the location posted to the internet from travellers. A picture is worth a thousand words.