Check for changes to an SQL Server table?

Unfortunately CHECKSUM does not always work properly to detect changes.

It is only a primitive checksum and no cyclic redundancy check (CRC) calculation.

Therefore you can't use it to detect all changes, e. g. symmetrical changes result in the same CHECKSUM!

E. g. the solution with CHECKSUM_AGG(BINARY_CHECKSUM(*)) will always deliver 0 for all 3 tables with different content:

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 1 as numA, 1 as numB
  UNION ALL
  SELECT 1 as numA, 1 as numB
)  q
-- delivers 0!

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 1 as numA, 2 as numB
  UNION ALL
  SELECT 1 as numA, 2 as numB
)  q
-- delivers 0!

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM 
(
  SELECT 0 as numA, 0 as numB
  UNION ALL
  SELECT 0 as numA, 0 as numB
)  q
-- delivers 0!

Take a look at the CHECKSUM command:

SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM sample_table WITH (NOLOCK);

That will return the same number each time it's run as long as the table contents haven't changed. See my post on this for more information:

CHECKSUM

Here's how I used it to rebuild cache dependencies when tables changed:
ASP.NET 1.1 database cache dependency (without triggers)