Multixacts wraparound monitoring

Hello,
How can we determine when an error of approximation multixacts wraparound?
According to the information from pg_class:
select datname,datminmxid from pg_database;
datname | datminmxid
----------------+------------
template1 | 347462426
template0 | 347462426
postgres | 347462426
zabbix | 467261307
db_3 | 291141939
db_1 | 388282963
db | 388282963
But when the vacuum/autovacuum starts up, an error occurs:
WARNING: oldest multixact is far in the past
HINT: Close open transactions with multixacts soon to avoid wraparound problems.
If I understand correctly, approaching Multixact member wraparound.
But how to understand when it comes exactly and what to do?
PostgreSQL version - 9.3.10, OS Debian 7.8.
Thank you.
Sorry, if I chose the wrong mailing list.

Kind regards,

Vladimir Pavlov