I read about tagged ethernet frames, which include the extra 802.1Q field, and understand the concept and operation.
One thing I wonder about is whether there is a way for a node receiving an Ethernet frame to distinguish between tagged frames and non-tagged frames.
I read in answers about Trunk ports where it's possible to have on untagged vlan on each port (e.g. https://networkengineering.stackexchange.com/a/10565/19642), so how does the switch know that the incoming frame is untagged? Is it just assumed based on the fields of the 802.1Q header not having sensible values?