If it's recent it will be G or N -- to be sure, have a look at the box it came in or the literature that came with it or download the manual from the maker's site.
Or the hard way, log in to the router's setup pages via your browser and see which options are offered under Wireless Setting, Wireless Mode (or similar).
For how to login to set up pages, see router manual -- you might as well go the hard way to familiarise yourself, anyway.