if you have the host username and password log in as host, and go to host->host settings, and it'll be listed under Dotnetnuke version. If you don't have those details, download the dotnetnuke.dll file from the bin folder, right click it, and click the version tab. It will list it there e.g. 3.2.0.30881 is 3.2 (only the major and minor numbers are important)

Cathal

another option is accessing the database, checking the latest entry in the versions table or check for either *.sql files in \database folder (DNN 1) or *.sqldataprovider files in \Providers\DataProviders\SqlDataProvider, the one with the highest number shall be the current one.

If you have any version after 3.0, you can upgrade easily, otherwise you should carefully check these forums, how to perform the upgrade. In the latter case you wil have to check any installed 3rd party modules for versions compatible with DNN 3.2 (Current version is 3.2.2 for ASP.Net 1.1 and DNN 4.0.3 for ASP.Net 2.0