The first thing that will be checked is pg_stat_statements. Now answer the following questions:

• How many concurrent queries are currently being executed on your system?
• Do you see similar types of queries showing up in the query column all the time?
• Do you see queries that have been running for a long time?
• Are there any locks that have not been granted?
• Do you see connections from suspicious hosts?

The pg_stat_activity view should always be checked first, because it will give us an idea of what is happening on the system. Of course, graphical monitoring is supposed to give you a first impression of the system. However, at the end of the day, it really boils down to the queries that are actually running on the server. Therefore, a good overview of the system that's provided by pg_stat_activity is more than vital for tracking down issues.

To make it easier for you, I have compiled a couple of queries that I find useful for spotting problems as quickly as possible.