With this function you can check to make sure that the Steam Cloud service is enabled for your game. It will return true if it is and false otherwise. Note that this does not automatically mean that you can use the Cloud functions as the user can switch off Cloud synchronisation from their Steam Client. You can check this using the function steam_is_cloud_enabled_for_account, but, even if it is disabled for the user (and enabled for the game), the functions will still work to store and retrieve data from a local copy of all files, it will just not upload them to the cloud on the game end, nor synchronise on the game start.
quota = steam_get_quota_total();
The above code checks to see if the steam cloud is enabled for the game and if so it gets the size of the storage quota and stores it in a variable.