Review sites on hotels are helpful but remember they are personal opinions and different types of hotels suit different people.  Several sites have these the most well known being TripAdvisor.

Online bookings - If hotel booking systems are showing no availability, check that you are using the hotel's own website rather than a third party agency site.  Hotels sometimes only give a few rooms to other companies and may have availability when you look on their own website or call them direct.

Comparing prices - Check whether the price is "per room" or "per person".  If bed and breakfast is included, or is it Continental or English breakfast.  Is there a hotel car park, or is there an extra charge for parking, is VAT included.  Remember you are paying for additional facilities as well as the bedroom.

Comparing quality - Anyone can put up a sign calling themselves a hotel.  If hotels are registered with the Visit Britain or the AA it means that they have to comply with specific standards. 

Hotels that are registered should always be clean, meet specific standards and are assessed annually.  Professionally compiled guides are arguably more reliable than the opinions of random individuals, whose tastes may vary.   Look for the logo.