STG44,
Firing pin - the firing pin would normally have the serial number stamped on the flat.
Handguard serial number - when you take the barrel bands off of the rifle you can lift off the upper wooden handguard and separate the barrel/receiver from the stock. The serial number should be stamped on the inside of the upper wood handguard channel and also on the inside of the stock channel.
The follower - is the metal piece that you see when you pull the bolt back that is in the magazine channel. The ammunition rides on top of this piece. It should have the last two digits of the serial number stamped on it. Also, the last two digits of the serial number should be stamped on the top side of the extractor tip that is seen when you pull the bolt back out of the receiver.
Barrel band spring - this is the metal piece that is on the right side of rifle in between the barrel bands. It secures the barrel bands in place. When you remove it you should see the serial number stamped on its top side.
Look on YouTube and you will see videos on how to take down your K98k to see it's inside parts. It is very easy to do, but never force anything until you learn how to do it properly.
You will have to find a reference document to tell you what parts on your 1941 rifle were serialized (Bruce's book has this). Once you check each one off you will know if your rifle is fully intact from the factory or if some of the parts have been replaced at some point.
Good luck in your education on your new K98k!