Well, there's no such thing as an INVISIBLE tag. The trouble is, there are a number of ways they could be hiding your counter is, and there's no good way to scan for all (or even one) of them automatically. My guess is that they're mostly using the CSS "display: none;" property. You could try to override this with your own display property (adding "important!" to override theirs), but that won't help if they've enclosed your counter in a div (i.e.) of their own.
Unless you have thousands of users, you might just be better off checking their pages manually.