Like RegDwight's comment, I'd say that you can't simply extend "check out" and have the sentence work. You check *something* out. You could say 

> "Check out this website for offers!"

or

> "Check this website out."

Those are correct, but they're not as concise as 

> "Check online for offers!"

Even better, you want to say

> "Check our website for offers!"

because it draws their attention to you in a more personal way, which is the point of advertising.