Replication data issues.

how to find out that the data is not delivering to subscribers for below mentioned concepts.  

1.      Data Is Not Being Delivered to Subscribers(how to find out)
2.      Data at the Publisher and Subscriber Do Not Match(how to find out)
3.      The System Has a Performance Issue(how to find out)
4.      Security Issues Are Preventing Data from Being Replicated(how to find out)
5.      A Merge Subscription Has Expired and Changes Must Be Uploaded(how to find out).

thanks in advance.