Difficulty joining a standalone HA pair

Hello guys, can anyone give me feedback about this question?

I am experiencing difficulty joining a standalone HA pair. How should I diagnose the issue? 

1)Use error messages recorded in syslog and debug log on both nodes?

2)User traffic capture on the first node (the active node)?

3)Use traffic capture on the second node (the node the customer is trying to join to the active node)?

4)Use traffic capture on both sides?