How to check if a Cassandra table exists

As of 1.1 you should be able to query the system keyspace, schema_columnfamilies column family. If you know which keyspace you want to check, this CQL should list all column families in a keyspace:

SELECT columnfamily_name
FROM schema_columnfamilies WHERE keyspace_name='myKeyspaceName';

The report describing this functionality is here: https://issues.apache.org/jira/browse/CASSANDRA-2477

Although, they do note that some of the system column names have changed between 1.1 and 1.2. So you might have to mess around with it a little to get your desired results.

Edit 20160523 - Cassandra 3.x Update:

Note that for Cassandra 3.0 and up, you'll need to make a few adjustments to the above query:

SELECT table_name 
FROM system_schema.tables WHERE keyspace_name='myKeyspaceName';

The Java driver (since you mentioned it in your question) also maintains a local representation of the schema.

Driver 3.x and below:

KeyspaceMetadata ks = cluster.getMetadata().getKeyspace("myKeyspace");
TableMetadata table = ks.getTable("myTable");
boolean tableExists = (table != null);

Driver 4.x and above:

Metadata metadata = session.getMetadata();
boolean tableExists =
  metadata.getKeyspace("myKeyspace")
    .flatMap(ks -> ks.getTable("myTable"))
    .isPresent();