Howdo I tell which kernel modules are required

You can usually find this information by digging through /sys if you're on a 2.6 kernel.

e.g.

$ ls -la /dev/input/mouse1   
crw-r----- 1 root root 13, 33 2010-03-08 15:56 /dev/input/mouse1
$ ls -la /sys/class/input/mouse1/device/driver 
lrwxrwxrwx 1 root root 0 2010-05-12 23:33 /sys/class/input/mouse1/device/driver -> ../../../../../../bus/usb/drivers/usbhid

So the driver in this case is usbhid. There might be a better/neater way of doing this but I find digging in sysfs usually gets the job done.

They are loaded based on hardware detection. In the case of your e1000e module the hardware in your machine, whether it is on board or a plugin card, will identify itself with a PCI vendor ID and device ID pair and those will be matched against the available modules and any module identifying itself with that ID will be loaded.

You can see the IDs associated with the e1000e module by running modinfo e1000e which will report something like this:

filename:       /lib/modules/3.9.5-301.fc19.x86_64/kernel/drivers/net/ethernet/intel/e1000e/e1000e.ko
version:        2.2.14-k
license:        GPL
description:    Intel(R) PRO/1000 Network Driver
author:         Intel Corporation, <linux.nics@intel.com>
srcversion:     28B371A0E50A24E26204016
alias:          pci:v00008086d00001559sv*sd*bc*sc*i*
alias:          pci:v00008086d0000155Asv*sd*bc*sc*i*
alias:          pci:v00008086d0000153Bsv*sd*bc*sc*i*
alias:          pci:v00008086d0000153Asv*sd*bc*sc*i*
...

Only with a much long list of alias lines, each of which identifies a specific PCI device that the module is able to support.

As devices appear the linux kernel will announce them to userspace using uevents, and the udev daemon will match the device IDs against the module database and load the correct modules.