rattle at idle, possible harmonic balancer, what else?

A few days ago on a cold morning, I noticed a rattle when I started the car. It's only there at idle, it goes away at all other rpms. Reading here on the forums suggests it could be the harmonic balancer on the end of the crank shaft. How can I confirm the diagnosis? Could it be something else?

I just don't want to waste $75 on a new part only to find out it's not the problem. The car is a 2004 PD BEW. Any advice is appreciated!

Dmf.........

Thanks pwrStrk, but I have a SMF (single mass flywheel). Can visual inspection identify a bad harmonic balancer?

It was the alternator pulley.