As already mentioned most obvious will be serial # area ( seen through "window" of SL-8 ) and the "marking plate" . Refer to G36 armourer manual ( if you have it ) on page 13 shows how there is far more info on the marking blate of G36 , such as proof house etc , and serial # ( unlike a SL-8 that has serial # in the window and only says model, caliber, made in bla bla bla )
If you really want to be sure, a FBI lower ( two position semi auto ) still has a " catch" as HK calls it. This is similar to an auto sear in AR . It holds catches the hammer after the sear releases it and only re releases hammer as two "ski bumps" on a G36 bolt carrier trips it as it closes ( as a M16 carrier has the extra meat to trip the auto sear )
So , a semi auto G36 also has a G36 bolt carrier that has these bumps. If you google G36 carrier you will see what I mean. These bumps also preven the installation of a G36 carrier into a SL-8 reciever
Therefore, a real G36 will have a G36 carrier, and two grooves machined down the length of the carrier rails to allow the G36 carrier actually fit
Takes seconds to pull takedown pins to have a peek inside