First, I should probably ask if it's against company policy to use IM programs from behind their firewall - and I'd assume that it is.
Anyways, if you run a program like
TCPView, you can see what programs have what ports open - and I think that might help you.
Just be aware: if using IM programs is against your company policy, even if you run them on non-standard ports, the network staff can still see the activity. I've been on the enforcing end of that rule a few times, and shocked a lot of people when I sat down with them and a few other security officers with detailed IM logs of what they've been talking about, and explained that they really need to stop using IM at work.