Wheel cylinder leaking or rear axle seal? How to tell the difference?

I pulled my drums and one side has some slight moisture inside. Nothing major, but definitely didn't look like the other side. Is there an easy way to diagnose whether it is the wheel cylinder of axle seal? I would think the gear oil smell would be obvious, but it's such a small leak that I'm not sure I would even smell it.

 

Pull the boots back on the wheel cylinders, if they have fluid behind them then they are leaking and should be replaced.

 

I was hoping it was this easy, thank you Dm93!

 

If those drums haven't been off for a long time it may not be that easy! Good luck.