That Jew would know that because he was not allowed to keep or bear arms, nor protect himself or his family, or to give testimony in court, or serve in the local militia.
He could be dragged from his home and tossed into the wilderness to be ravaged by wild animals or the peasantry at will.
It's not so much that we have rights because we can arm ourselves ~ rather, it's that we cannot enjoy any of those other rights without that one, single, solitary right.
Might note that this is also a "communal" or "group" right ~ not just an individual right.
We, as a group, e.g. a town, a city or just a settlement in a "compound", can arm ourselves, and presumably can require all our members to arm themselves or go elsewhere.
Kennesaw, Georgia, did just that.