Please see the following links for information regarding recognizing problems within the hive.
Tools for Varroa Management
Sanitation and Best Practices to Prevent the Spread of Pests and Pathogens in Honey Bee Colonies
Foulbrood Information
Diagnosis of Honey Bee Diseases
Disease and Pest Sample Submissions
Preventing Pesticide Poisonings of Honeybees
Bee Health information from eXtension.org