Here are some simple queries that might be helpful for you.
To check number of open connection in SQL server: 
- SELECT DB_NAME(dbid) AS DBName, COUNT(dbid) AS NumberOfConnections, loginame AS LoginName  
 - FROM sys.sysprocesses  
 - WHERE dbid > 0  
 - GROUP BY dbid, loginame;  
 
 
To check the number of queries running currently on your server:
- SELECT sqltext.TEXT, req.session_id, req.STATUS, req.command, req.cpu_time, req.total_elapsed_time  
 - FROM sys.dm_exec_requests REQ  
 - CROSS APPLY sys.dm_exec_sql_text(sql_handle) AS sqltext;  
 
 
Find all dependencies of a table in Sql server 
- SELECT referencing_schema_name, referencing_entity_name, referencing_id, referencing_class_desc  
 - FROM sys.dm_sql_referencing_entities('dbo.Clients', 'OBJECT');  
 
 
Shows all the sessions that are currently established in Sql server. 
- EXEC sp_who2  
 -   
 - //Or  
 -   
 - EXEC sp_who  
 
 
Shows all ACTIVE sessions that are currently established in SQL server. 
- EXEC sp_who 'Active'  
 -   
 - //Or  
 -   
 - EXEC sp_who2 'Active'