How can I tell if a tag has been pushed, or not?

Here's the problem:

I expect that users, in their own private repos, will from time to time create tags that should not be pushed. Sometimes, tags that should be pushed will be created.

I could require that the "public" tags follow a specific convention (start with "release", or something). Then, the scripts all my developers use could use that and push only certain tags. However, over time there could be a large number of them. It seems undesirable to push each tag each time a push is done. So, how can I tell if a tag has already been pushed? Is there a way?

Thanks.