1864 Indian Head Cent

The Copper Nickle is Much thicker and heavier than the Bronze and has a lighter colored look. The Bronze is the thinner lighter one.
The Copper Nickle weighs - 4.67 grams
Bronze is - 3.11 grams
On the bronze, keep an eye out for a SMALL L between the bottom feather and head

There are actually THREE 1864 varieties. The copper nickel variety is thicker, heavier as noted above and lighter in color, almost a washed out whitish brown appearance.

The two bronze varieties are thinner and weigh less and are the same size and weight as other cents from 1864 to 1982. One vairety has L in the ribbon and is quite valaubale because it was minted less, plus the L easily wore off even at grades of F. The other variety is the common and had no L in the ribbon. These two can be distinguished as noted by HezelsCoins above, by the bust. 1864L has a pointed bust, while 1864 No L has a rounded bust. So many collectors with a keen eye can cherry pick the 1864 L by the bust diagnostic even if the L is not showing.