MySQL - Cómo verificar si START TRANSACTION está activo

Puede crear un procedimiento que aproveche un error que solo puede ocurrir dentro de una transacción:

DELIMITER //
CREATE PROCEDURE `is_in_transaction`(OUT $transaction bool)
BEGIN
    DECLARE oldIsolation TEXT DEFAULT @@TRANSACTION_ISOLATION;
    DECLARE EXIT HANDLER FOR 1568 BEGIN
        -- error 1568 will only be thrown within a transaction
        SET $transaction = true;
    END;
    -- will throw an error if we are within a transaction
    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
    -- no error was thrown - we are not within a transaction
    SET TRANSACTION_ISOLATION = oldIsolation;
    SET $transaction = false;
END//
DELIMITER ;

Pruebe el procedimiento:

set @within_transaction := null;
set @out_of_transaction := null;

begin;
    CALL is_in_transaction(@within_transaction);
commit;

CALL is_in_transaction(@out_of_transaction);

select @within_transaction, @out_of_transaction;

Resultado:

@within_transaction | @out_of_transaction
--------------------|--------------------
                  1 |                   0

Con MariaDB puedes usar @@in_transaction