Yes, you are able to monitor a DB in the PLC. DB's are added either by the "Add block" process, or when a block, like a Function Block is added. DB's are listed in the Program Blocks area or in the System Block item in TIA Portal. In SIMATIC Manager, DB's can be monitored in the Blocks entry under the S7 Program folder. In both cases, if you go Online and open up the DB, you will be able to monitor the block values.