Weight is the fastest test
Aluminium is roughly a third the weight of copper for the same volume. A reel that two people can lift comfortably is usually aluminium; the same footage in copper needs equipment.
The cut end
Copper is red-brown and stays that way; oxidised copper darkens to brown-black. Aluminium is silver-grey and oxidises to a dull white-grey. Fresh cut ends are unambiguous.
Read the print
Many constructions state the metal or use an AL designation, and aluminium is common in SEU, URD, XHHW feeders and ACSR. Where the print says nothing, the cut end decides.
Why it matters commercially
Copper carries a high metal floor, so even unidentified copper has scrap value. Aluminium has a low metal floor, which means the cable premium over scrap is proportionally larger — and identifying the construction matters more.