I'm guessing that you want to script this to check all tables.
You can append a tablename to sp_spaceused to get the usage of a specific table. If you need to do multiple tables build and execute a script (eg. "select 'exec sp_spaceused ''' + name + '' from sysobjects where type = 'U'" then execute the resultant script).
Note too that you may need to append the the 'updateusage' parameter if you haven't done that for a while.