It's been a while since I played with this stuff, but I'm pretty sure you can use the TDS tool to monitor the trunks in question and see the calls go out. It will be much easier at a quiet time because usually the provider will set any incoming calls to the PRI to either come in top down or bottom up (circular is also an option), and then the system designer will set outbound calls to the opposite direction. You should be able to see that setting in the trunk group TGACC (or BUEND).
That way you can set up TDS to only monitor the top few or bottom few trunks and you will be able to catch the call when it goes out with less effort. Set it up, dial a test call and watch. Otherwise you have to set it to monitor them all and try to figure out which one the call is going out on (assuming they are routing correctly).
You may also be able to do an ISDN trace (also in the TDS tool) on the D Channel for that PRI, and then you can see the call setup information scroll by for everything in the same place. Like I said, it's been a while since I had to do it.