Under USC 10.311, if you are a male between 17 and 45 and not in the armed forces, you're in a militia and it is your own responsibility to procure arms and training in case you are ever called up. Therefore, you are indisputably in a well-regulated militia, as its composition is codified under US federal law.
http://www4.law.cornell.edu/uscode/10/usc_sec_10_00000311----000-.htmlTITLE 10--ARMED FORCES
Section 311. Militia: composition and classes
(a) The militia of the United States consists
of all able-bodied males at least 17 years of age
and, except as provided in section 313 of title
32, under 45 years of age who are, or who have
made a declaration of intention to become, citi-
zens of the United States and of female citizens
of the United States who are commissioned of-
ficers of the National Guard.
(b) The classes of the militia are--
(1) the organized militia, which consists of
the National Guard and the Naval Militia;
and
(2) the unorganized militia, which consists
of the members of the militia who are not
members of the National Guard or the Naval
Militia.