Registered trade-marks are designated with the ® symbol, and regular trademarks with the symbol.
Even if something doesn't have the symbol next to it, if it's it's a name or logo that's used to identify a product that is being traded, then it's still a trademark nonetheless.
e.g. If I copied the gamedev.net logo and sold my own forum subscriptions using that logo, then the real gamedev.net could sue me for trademark violation, even though they haven't used the symbol.
Simply putting "international trademark registration" into google will give you information about how much registration costs -- Likely between $100-$1000 per country.
Generally only very big brands, like Coca Cola use registered trademarks.
Note that regardless of whether a trademark is implicit (no symbol displayed), explicit ( symbol displayed) or registered (® symbol displayed), if someone violates your rights by copying your name/logo, the only thing you can do is hire a lawyer and start a lawsuit against them...