Most legitimate companies have ongoing work performed to their websites and of course with each change or update new copyright dates attach. If you have questions about any website you can check with whois.net or similar on line resource to see who 'owns' the website address. While sometimes you will get information that it is godaddy.com, there are some investigations you can take further.
If you are uncomfortable looking for websites out there, check the list in the first post of the first thread of the Mystery Shopping Company Discussion section or check the list of companies on the Volition.com board. The links you find should take you directly to the company's sign up website. While other lists will pop up from time to time in various places here or elsewhere, your best bet is to go with a list from an established entity that has no reason to spoof a site for you.
Once you are at a site, use your common sense. Look at the URL bar. Does it say 'https://'? If it does you are dealing with a secure website. Some browsers will put a little lock in the URL bar or in the bottom corner of your screen so you know you are on a secure website. Very few legitimate companies do not offer you a secure website to submit your personal data any more. (A few years ago this was NOT always the case and I remember corresponding with one company by email that seemed surprised that anyone could possibly be concerned about putting their personal data on an unsecured site.) "We don't know of anyone ever having a problem," no longer cuts it.
And of course, the safest bet of all is to only sign up with companies that YOU have found on a safe list such as from here, Volition.com or the MSPA. If you applied to a company you saw on Craig's List or in the newspaper and they come back to you looking for personal information be sent to them by email, run the other way fast! You are in the sites of a scammer more than likely.