How to check the isolation level

SQL Server supports all four ANSI standards isolation levels. They are;

1. Read Uncommitted (the lowest level, high concurrency, less blocking, more currency related issues) 
2. Read Committed (default)
3. Repeatable Read
4. Serializable (the highest level, less concurrency, higher blocking, no concurrency related issues) 

Other than the above four, SQL Server introduced additional two optimistic concurrency control isolation levels based on row versioning technique;

1. Read Committed Snapshot
2. Snapshot isolation level

DBCC USEROPTIONS command can be used to see the current isolation level in SQL Server.