Some tips for deciding whether an information source is trustworthy and usable:
Currency: It's generally best to aim for newer information sources, but keep in mind that a lot of basic chemical properties were established a long time ago!
Relevance: Is it the exact property you're looking for? Are the units right, or do you need to convert?
Authority: It's a good sign if the info is coming from an expert in the field, a major research institution or government agency, or a chemical industry provider.
Major reason why Wikipedia isn't the most reliable source - it's difficult to determine authorship.
Accuracy: Is a source cited? Can you find any other sources corroborating this one?
Purpose: Who is the info for? Is this data for working chemists, or part of a high school lab manual?
Google doesn't index every page on the Internet. If you're looking for a specific piece of numeric data, you might not find the answer by Googling it individually. A better strategy is going straight to a reliable data source and searching by the compound name (for ChemSpider) or the type of data you're looking for (CRC Handbook).
You can use Google to guide your thinking about methods. Try various combinations of these phrases: