devmgmt.msc and look under Ports. It should tell you the LPT port number.
From the Wikipedia article, here's the IRQ's and x86 port ranges of the LPT ports. Use the first I/O address in the range correspnding to the desired LPT port, or just try them all (0x378, 0x278, 0x3bc).
LPT1 IRQ 7 0x378-0x37f
LPT2 IRQ 5 0x278-0x27f
LPT3 IRQ 2 0x3bc-0x3bf
If your LPT->USB hardware adds the port as a LPT4 or higher, you'll need to dig further in any available device documentation to find out the address (LPT's over 4 were extremely rare on actual PC's, if they ever did exist...)
Good luck. Sounds like you are using an old DOS program that expects to talk directly to the LPT ports - not sure how the NTVDM under XP/Vista/7 (if that is what you are using) will work in that situation with a LPT->USB converter. You may need an old PC with an actual parallel port to accomplish what you are trying.