The source of the data is a major indicator of its quality and reliability.
Reputation of the Collector: Is the data set from a reputable source, such as a government agency (e.g., U.S. Census Bureau, Bureau of Labor Statistics), a well-known academic institution, or a respected research organization (e.g., Inter-university Consortium for Political and Social Research - ICPSR)? Data from these sources are generally considered more trustworthy because they follow established collection methodologies.
Potential for Bias: Consider the purpose for which the data was originally collected. Was there a specific agenda or commercial interest? For example, data from an industry group might be biased in a way that data from a neutral government agency would not be. Ask yourself, who collected this, and what did they hope to achieve with it? This will help you spot potential bias before using the data for analysis.
Documentation: High-quality data sets come with comprehensive documentation, often in the form of a codebook. This document should detail the study's methodology, sampling procedures, data collection instruments, and definitions of all variables.