The two markets
Copper cable is valued largely on contained copper plus a premium for a usable construction. Aluminium is valued on contained aluminium, which is a much lower number per pound, but the multiplier for a legible construction is often larger because so much aluminium is scrapped without being read.
Identifying the conductor
Aluminium is lighter, softer, and silver-grey at a cut end; copper is heavier and red-brown. Many aluminium constructions print 'AL' or 'aluminium' in the legend, and service entrance and URD cable are frequently aluminium. A magnet is useless here — it identifies steel armour or ACSR cores, not the conductor.
Where aluminium wins
URD triplex, SEU service cable, XHHW feeders and ACSR overhead conductor are common in aluminium, frequently in long lengths, and often still on tagged reels. Priced as cable rather than scrap, those lots usually clear the scrap number by a wide margin.
Where copper wins
Welding cable, THHN branch wire, control and instrumentation cable, and bare copper grounding conductors carry a higher metal value and a reliable scrap floor, so a copper lot is rarely worthless even when the construction is unreadable.