How can I tell if another device is synced with my phone?

I'm not sure what you are asking. An iPhone can only sync with one computer, and cannot sync with any other devices other than a computer. So if it syncs with your computer it is not synced with any other.

If you mean can it share content, it can share with all devices that are under your Apple ID. You can see what is registered with your Apple ID by going to iTunes, clicking on the button with your Apple ID in it, logging in, and then click the Manage Devices link.