We can check open connection of our database using inbuilt stored procedure.
You can use sp_who stored procedure. It provides information about current users, sessions, and processes in database instance.
You can also use undocumented stored procedures sp_who2 which gives you more detailed information
You can also use below query for it
SELECT DB_NAME(dbid) as DBName, COUNT(dbid) as NoOfConnections, loginame as LoginName FROM sys.sysprocesses WHERE dbid > 0 GROUP BY dbid, loginame