You can't really tell if a website is legitimate or not just by looking at it. In fact, it's the same thing for every (not widely used) internet service, not only for forex signaling services.
They will be asking for money to access the private part of the website (or email subscription...) to access the service. And since you can't be sure of the quality of the service before paying, you can be scammed.
What you can easily do, is try to find the information of the society holding the website. If the site is legitimate, there should be an "about us" section somewhere which discloses information about the company (like registration number or name). With those references, in some countries, you can access a database of registered companies, thus trying to find is there is really something serious behind the website (I often do that before an interview to get financial information and the owners' names).
But, the owner of the services could try to impersonate a company by putting false information, so, just ask the company more information about the signaling service, if its not theirs, they will tell you.
Also, even if the company is legitimate, that does not mean that they won't try to scam you (like Madoff). But if they do, at least you know what/who is behind the website, which makes it easier to find them in case there is a problem.
Also, most scam websites don't have an address on the contact us page. (Most don't have a "contact us" page at all.) Be aware of such websites. Never give in to websites that promise huge return on investments. These are scammers and they try to tempt you and then loot you. You might end up losing money and personal financial information like your credit card number, bank details, etc.