How to determine if a postgres database needs to be vaccumed? Given a PostgreSQL database what command can be used to determine if the database needs to vacuumed? 

UPDATE I am working with client who is evaluating postgres but I am not a DBA, I just happen to be more experienced with postgres than they are. They are getting update times of several hours in their tests with 500K rows in a table. They had not done any changes to their GUCS for shared buffers, work_mem ... etc I just went through with them and had them do that. In some of their tests postgres reports 3674 seconds for updates that take 2.66 seconds in oracle and 33.8 seconds in MySQL. I want to be able to answer the question would Vaccum make a difference to their tests, but to be able to get real data back to them about the impact of vacuum on their tests.

Postgres 9.2 on CentOS 6 Intel Dual Process Quad Core Xeon K5570 8GB RAM
There is no single parameter that determines, whether vacuum is necessary, but it can be quite interesting to look at the thresholds that postgres uses to trigger autovacuum. 

Details and a query to do so can be found in another thread: Aggressive Autovacuum on PostgreSQL.