Question

how do you identify feet as a derived or fundamental unit

Answer 1

Do you mean "base unit" instead of "fundamental"? O_o
Yes, the foot, like the meter, is a base unit of length.

Derived units are made from base units. (i.e.: Ohm, Volt, Tesla, Watt)

Base units (there are seven):
http://en.wikipedia.org/wiki/SI_base_unit