Either by booting into single user mode (enter s in the options field during boot), or use a cd/DVD in rescue mode.
Edit: orr a live CD as BenderBendingRodriguez says (we are fighting being first to answer you, see what you did to us
).
When boot is finished and you are loged in in the console after the boot (from live CD or whatever), mount the roor partition of your normal system somewhere, then you go there with cd and see what you have in etc/.