Decoding interface flags (ifconfig)

[Clint Pachl]

I'm curious if the flag bits, shown for each interface with ifconfig(8), 
can be decoded in order to reveal the characteristics of NICs, such as 
hardware RX/TX checksums and VLAN.

So far I have searched:
netintro(4)
ifmedia(4)
inet(4)
sys/net/if.c
sys/dev/pci/if_em.c

But haven't found anything definitive. Can someone explain or point out 
some source or man page?

-pachl