Return "c" and indices of a variable c[i,j] in Mathematica

Another possible way using Level and Heads.

Level[c[i, j], 1, Heads -> True]
(* {c, i, j} *)

A simple approach is

c[i, j];
{Head[%], %1[[1]], %[[2]]}
(* {c, i, j} *)

Slightly more generally, use

f[x_] := Join[{Head[x]}, List @@ x]
f[c[i, j]]
(* {c, i, j} *)

using pattern-matching/deconstruction

c[i, j] /. a_[b_, c_] -> {a, b, c}

or define your own function:

f[a_[b_, c_]]:={a, b, c}

f[  c[i, j] ]

{c, i, j}

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