It helps if you are a contortionist. If the tag is still there, it will be on the back side of the ignition lock. You have to look up from under the dash at the lock assembly. It will be on the part that goes over to the steering column and on the part facing the dash itself.
If you are really lucky, you can read it backwards with a mirror.
It's a white tag that is about 1/2 x 1" and will have 4 numbers on it.
Here is a good picture of what you are looking for in this link.
http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/617458-never-used-911-ignition-sw-assembly-w-2-keys.html
Absent that, pull your left door handle and take it to the lock smith.
Any good lock smith can pull the tumblers and know what the cuts are.