It should say in the BIOS. For example if your operating system hard drive (C:) is on IDE 0 it will say on the BIOS Primary Master and if you have any CD-ROM's or DVD-ROM's on IDE 1 it will be listed under Secondary.
A little more info will help....'two disk drives'...do you mean HDD and a CD/DVD or do you mean two CD/DVD Rom drives ? If the former then the HDD is usually on one IDE connection and the CD/DVD is on a separate IDE connection. If the latter usually the top drive in the case is the master and the one below is the slave because of the way the IDE cable has to connect. Check by R click My Computer - Properties - Hardware tab - Device Manager. Click + beside DVD/CD Rom Drives. Double click first drive and it should show Location 0 (as Master) Double click second drive and it should show location 1 (as Slave)