Level3/Comcast routing question (not related to the peering dispute)

Customers of ours on Comcast are experiencing poor
throughput to us when whatever location they're at
takes a route to us via Level 3; Level 3 being
one of our upstreams.

I've set a community of 65004:7922 which is supposed
to tell Level 3 to prepend four times to Comcast
for our AS but the only route server I know of in
comcast land (route-server.newyork.ny.ibone.comcast.net)
is not showing the prepend.

Is there another way for me to determine if Level 3
is really doing the prepend other than opening a
ticket and asking them? I did try
http://www.level3.com/LookingGlass/ and plugged in
one of our IP's and see "Prepend_4_to_AS7922" so
could they be prepending and comcast overriding
or ignoring it? We have peering with another
provider that's directly connected with comcast so
I'd like to have traffic from them use that one instead.