Well, the first thing you will want to do is call Mesa Boogie and ask to speak with someone who can identify the date of manufacture of your amplifier by the serial number. You can also try pulling the chassis to see if they wrote it in there. The older 2 channel versions have a Month/Year date inside them.
The 2000 and 2001 models don't have a little label above the Solo Output Volume and the Overall Loop Master Volume that says "Loop Active". These are the two knobs to the far left. From 2002 to the Present, there is a label above these two controls that says "Loop Active". You can see it in the pictures below.
This is a 2000 or 2001 model
http://i9.ebayimg.com/01/i/03/83/1f/8e_1_b.JPG
This is a 2002 to 2005 model
http://i20.ebayimg.com/01/i/03/92/41/ab_3.JPG