How can I check to see if my Mac is running in dual channel mode?
Pretty simple.  Sorry if I go through stuff that you know but... you never know.

Dual Channel

Dual channel RAM is a chipset thing. DDRx is a RAM type.  DDR2 and 3 are NOT Dual nor Tri Channel RAM...  Your motherboard/chipset determines your Dual Channel ability.

So look up your motherboard's specs or use Crucial's System Scanner which is good stuff.  So if you are not sure if your system supports DC you can verify that there.

Dual Channel RAM

"If you have a dual-channel platform and you want to take advantage of the performance gain it offers, our advice is to simply purchase your DDR or DDR2 memory in pairs. However, be very careful to order two modules with the exact same specifications; the modules must be identical to each other to perform correctly. "

so Dual channel only works with two SO-DIMMS with the EXACT SAME specs 

Here is more from macrumors.com/guides:

"All Intel Core Macs support dual channel memory access if matching modules are installed. The customary estimate is that this gives a 6% - 8% real world performance benefit. The modules do not have to be the same brand. That means it is quite possible but not 100% guaranteed, that adding a 3rd party SODIMM to an Apple supplied SODIMM of the same size will make a matched pair."

Verification

you can click the Apple Menu at the top left, About This Mac then More Info then click System Report... click Memory on the left hand side.

If you want to get nuts you can use TechTool Pro (not verified on Lion) and get a very detailed report which I've attached images of below of my RAM.  This is just running a terminal command I'm sure - I just don't know it.

Point being is, there is not an indicator because ... if your system supports it, and the RAM is bought in identical pairs... its running Dual Channel.

Best of luck.  

In the pics, all I'm doing is looking at the numbers besides the S/N and for peace of mind making sure they match.