Assessing the reliability of any website is very important, especially since the Internet has no doubt become the most popular and utilized source of information in the world. Because of that popularity, though, it is essential not to assume everything found on the Internet is true.
The link below includes a helpful list of specific things to look for when assessing the credibility of any website, but your question refers specifically to .com sites, and you are right to be concerned.
A .com site is a commercial site, and that means the primary motivation for a .com site is making money. In general, think of these sites as advertisements for goods and services and treat them accordingly. Do some extra research if you are doing any shopping for products on these sites.
Some .com sites are not really advertising anything, though they do contain advertising. For example, some city or business locator sites are .coms, and they serve as a kind of modern on-line phone book. These sites list businesses, including their addresses and telephone numbers, so it is in their best interest to be correct and reliable; however, unless they are maintained regularly, these sites often contain errors due to old businesses closing or moving and new businesses opening.
In my search for a website assessment link to include for this question, I discovered several which were .coms. These are as likely to be as true as any of the .edu sites on the same subjects, though of course one must always do some checking. If you find the same information on a relatively straightforward, factual topic in several places, the .com might be perfectly accurate and helpful. It is impossible to make the judgment that every .com website is either trustworthy or untrustworthy. Some are and some are not, and the only way to know with reasonable certainty is to assess them.
For more specific criterion on website evaluation, see the link below. In general terms, though, awareness is the best prevention when it comes to gathering information from the Internet. Because .com sites have something they want to "sell" you, the buyer (which is you) must beware.