Try to find some information on the site that tells you who the author is. This could be in an ‘About us’ section, or in the contact details, if any are given. Does the author have some authority in terms of the information on a site? For example if it is a medical site, is it owned by a hospital or written by a doctor? Is the author a well known expert? Do they mention a number of books they have published on the same topic?
If an author is better qualifed, in terms of education or experience, in a particular subject area, it is more likely that the information will be reliable. Check that what they have written is objective: in other words, that it isn't biased to one particular viewpoint. However, sometimes those sites can have their uses too.