You can use the dm_exec_function_stats (in SQL Server 2016) to check detailed execution details of a function. These include:

execution_count - the number of times this function has been executedlast_execution_time - how long did the function take to execute

Here is a nice example of using dm_exec_function_stats

SELECTDB_NAME(database_id) AS DBName,OBJECT_SCHEMA_NAME(OBJECT_ID, database_id) + '.' + OBJECT_NAME(OBJECT_ID, database_id) AS FName,STATS.execution_count,STATS.total_elapsed_time / STATS.execution_count AS ElapsedTime,STATS.last_execution_time,STATS.max_worker_time,STATS.max_physical_reads,STATS.max_logical_reads,STATS.max_logical_writes,TEXT.TextFROM sys.dm_exec_function_stats STATSCROSS APPLY sys.dm_exec_sql_text(sql_handle) TEXT