Sometimes the slot numbers are silk-screened onto the circuit board itself, but usually the best way to know slot numbering is looking at the documentation that came with the computer and with the expansion chassis. There are also some software that can probe the hardware and display slot numbering, and sometimes even what type of card is installed into the PCI slot. An example of these would be the Apple System Profiler, and Digidesign's Digitest. Note, however, that not all types of PCI cards will be properly recognized by all applications. If you need to find this information with certainty, you should visually inspect the computer and expansion chassis.