If all replicas of one or more blocks of a file become unavailable, a file is considered corrupt and any attempt to access this file will lead to exception.
To check health of hadoop filesystem like Linux hadoop has "fsck" command. 
fsck generates a summary report that lists the overall health of the filesystem. HDFS is considered healthy if—and only if—all files have a minimum number of replicas available. 
 
[hduser@nn.zettabyte.com ~]$ hdfs hadoop fsck /

This command checks health of entire filesystem, it can also be limited to a specific directory.