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

"Check out this website for offers"

if you really wanted, but

"Check online for offers"

or

"Check for offers online"

both accomplish what you're asking about and are more concise.

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.