How to check whether the row was modified by this transaction before?

In a BEFORE UPDATE trigger I need to know whether the row was previously 
modified by this transaction. Is it safe to use xmin and txid_current() 
for this purpose (xmin is 32-bit txid type but txid_current() returns 
64-bit bigint).

CREATE FUNCTION test_trigger()
RETURNS TRIGGER AS $$
BEGIN  IF OLD.xmin = txid_current() THEN    -- Do something.  ELSE    -- Do something else.  END IF;
END;
$$ LANGUAGE plpgsql;