If you aren't sure of the material the weight will give them away.
Aluminum tanks of 20, 30, and 40 pound capacity have a tare weight of 14, 16, and 21 pounds respectively. It is stamped on the collar after the letters TW and may vary up to a pound or so depending on the vintage and manufacturer of the tank.
The steel ones have a tare weight of 18, 25, and 33 pounds for the same sizes.
If the tank has a float gauge it may weigh a little more.