Ethernet II Trailer identification

Hi there, I am wondering seeing ethereal interface if it is possible only
with the information of the Ethernet II fields to know if it has a trailer
and what is the length of that trailer. How can ethereal find the legth of
that added trailer to the end?.I can't find any way of doing it with the
exception of asking the upper protocol the legth of its data and seeing if
that legth of the encapsulated frame matches with the legth of the data
field of the Ethernet II frame.
The most i can do only with the ethernet ii field is seeing if the data part
of the frame has a length of 46 bytes and if it is the case i know there
could be trailer bytes added to it.
Anyone can explain me this to me a little more?(Or maybe give  me a
reference)
Thanks!!!

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