We have an elasticsearch cluster 5.6.12 which xpack didn't installed on it, new indexes created on this cluster regularly, I found the index alias and I want to see the rollover condition of that index alias but I can't find any API for this purpose at this version, can any one help me?
Welcome to our community! 
What do you mean by rollover sorry?
That's very long past its EOL date, please upgrade with some urgency.
Thanks,
Rollover creates a new index when the existing index is considered to be too large or too old.
Ok, that is not available in 5.X sorry.