What to do in times of data inconsistency in Master-Master Replication in MySQL?
First point is that you should only write to one Master at a time, with the other set as the 'backup' option in HAPROXY.

This will prevent you from writing to an out of date master (unless there is a problem with the main one, in which case you probably don't have an option anyway).
By writing to both masters simulataneously you are opening yourself up to a whole load of problems. If you need to write to both you should probably look at Group Replication or Galera Cluster.

Second point, I assume you have taken precautions such as setting the auto_increment_increment and auto_increment_offset values on the two databases? So that in the event you do write to them both (during testing failover etc) you won't get duplicate entries on your auto_increment values.

If you want to test for Seconds behind (and you have Linux servers) you can use a simple script adapted from here: http://sysbible.org/2008/12/04/having-haproxy-check-mysql-status-through-a-xinetd-script/

We use it to check various values to determine if a slave database is up to date and available for use.