How can I determine which clearinghouse an insurance provider uses?

-My boss (at an independent pharmacy) has me going through some old records to fix some problems we've had with payments from secondary insurances of medicare patients. I'm spending a lot of time on the phone with insurance companies to verify payment records, but its often a shot in the dark to find out which clearinghouse is used by which provider. Is there any online reference I can use to find these, or a more direct way to get the info from the providers?I work more normal medical claims and not pharmacy, but if you ask the providers they can usually tell you (or ask their for their EDI dept). However, please keep in mind, many clearinghouses, including one of the larger ones Emdeon, will not speak to anyone but software vendors that submit to them and payers.