Depending on the exact 3600 model you have, there is some info on:
http://www.cisco.com/warp/public/cc/pd/rt/2600/prodlit/kaos_ds.htm
If your IOS supports it, then a mere show ver would list that you have an encryption module on your router, otherwise a physical inspection of your router is another alternative.
Again, depending on the exact model of your card, the
first release of the AIM-VPN was around mid-2001. There was a different encryption module available prior to this.