After you've reconfigured and compiled the kernel with PLIP support enabled, when you boot the system, if the kernel supports PLIP directly, or when you load (later, see below) the PLIP module if you compiled PLIP as modules, you should get something like this (numbers may differ):
NET3 PLIP version 2.2 email@example.com plip1: Parallel port at 0x378, using assigned IRQ 7.
Depending upon your klogd and syslogd configuration the plip message could have been stored in your system log files: don't panic if you don't see the above message. If you compiled PLIP as a module and lsmod shows that the plip module is loaded, then it's enough.
Please take notice of the interface name. Usually is plip1, but may be plip0 or even plip2, plip3, and so on. It depends on the IO Address.