First, assume RG6 you find in a store is copper clad steel rather than solid copper unless you can determine otherwise. Steel is cheaper to make and buy.
If you have a strong enough magnet, you can feel the magnetic attraction through the outer insulation and shielding - so you can determine copper clad steel without cutting away the outer material.
If you do cut to expose the center conductor, then instead of using the magnet, I would just scrape a bit of the copper surface on the center conductor and see if it is copper or steel underneath.
Carl