People have set up web sites for Sirius where they log every song playing. So you can look at a grid and see everything now playing. I think they do it by monitoring radios that have a USB connection. As usual, the peeps are ahead of the actual companies in providing info online. They also built a better online player for the channels that are streamed, and a song search tool. Here's one grid, there are others I think:
http://www.itsonsirius.net/
I haven't kept up with the latest, but this site used to have it for Sirius. Checkout the stuff at the bottom on the knowledgebase, under "Digital Expert Network":
http://www.siriusbackstage.com/kb/Knowledge_Base
But by far the most up to date info is in the forums, not the wiki, same site.
Another site I used to visit...
http://www.dogstarradio.com/
And this one comes up on Quantcast*...
http://siriusbuzz.com/
For XM the main place to go is
http://xmfan.com/ ... maybe
http://www.xm411.com/
Or check out what the actual company websites have these days.
http://www.orbitcast.com/ covers both XM and Sirius.
*Quantcast. Seems like an interesting way to find links. Plug in one URL, and it gives you the most popular related URL's. Ad cookie data mining at work, I guess...
http://www.quantcast.com/ ... the stripped down version I stumbled on is here...
http://4.78.243.26/