You can limit a Google search to specific websites by entering site: and specifying the domain e.g. cancer site:gov.au will look for cancer information on Australian government websites.
The following questions can help you decide if it is a credible source:
Who is the author/owner of the site?
Is it a government agency? Commercial company? Organisation? Personal webpage? What authority do they have on the subject?
What is the intent of the website?
Are they trying to sell a product or service? Are they putting forward a particular viewpoint on the subject? Is there any bias or conflict of interest?
Is the content current?
Has it been published recently? Is there a copyright or last updated date on the website?
Is supporting evidence provided?
Are references/sources of information included to support claims made on the website?