How can I tell which of my two chinchillas is the aggressor when I'm trying to introduce them?

The cage within a cage method works best if you put one in the smaller cage for 24 hours than switch. In that case it will not matter who is being more aggressive. I know the cage within a cage method which was developed by Peggy aka Tunes on a href="http://www.chins-n-hedgies.com" rel="nofollow"http://www.chins-n-hedgies.com/a is listed on there in detail. If one is being the aggressor usually the more aggressive one stands out. I also trim off the whiskers of the more aggressive animal when doing introductions you could give that a try as well.