The following SQL statement is an easy way to monitor how many records are stored in each partition for a given partitioned table (the interactions table in this example):
SELECT p.partition_number, fg.name, p.rows FROM sys.partitions p INNER JOIN sys.allocation_units au ON au.container_id = p.hobt_id INNER JOIN sys.filegroups fg ON fg.data_space_id = au.data_space_id WHERE p.object_id = OBJECT_ID('interactions')
This produces results similar to those shown in the following table.
The table shows that each partition contains a single record. If you insert a new record and execute the above statement again, it will show which partition the new record has been placed in, verifying your partition function and scheme.