Memory used by SQL Server

Memory or RAM utilization by SQL Server

Here is the Query:

SELECT 

cntr_value/1024 as 'MB used'

FROM

master.dbo.sysperfinfo

Where

object_name = 'SQLServer:Memory Manager'

and counter_name = 'Total Server Memory (KB)'

or 

DBCC MEMORYSTATUS 

Apart from this you can use Prefmon with counters name SQLServer:Memory Manager, Total Server Memory (KB)

And if server is only used for SQL Server then in task manager check the memeory out of that only some percent

is used by your OS and rest is by SQL Server.