that depends where you live. look at http://www.ukfree.tv/starthere.php
then click "tv reception" and enter your postcode. this will bring up details of your local transmitter including (at the bottom) details of channels used by each tv station. click on bbc2 & the resulting map will tell you the frequency.
sorry missed out a step. when you see the map with the transmitter on (from your postcode), click its name. that brings up the page with the channel detail on.