Efficient way to test if all the elements in a list are Integers?

s = {1,2,3,3,5,6,3};

I would write:

And @@ (IntegerQ /@ s)

EDIT

With V10 we can use:

AllTrue[s, IntegerQ]

True

NoneTrue[s // N, IntegerQ]

True

VectorQ is the best way I know. Here are three types of input, packed array of Integer, an (non-packed) array of Integer, and an array of not all Integer.

packed   = RandomInteger[10, 10^7];
unpacked = Flatten@ConstantArray[{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, 10^6];
nonint   = ReplacePart[packed, 10^6 -> 1.5];

Developer`PackedArrayQ@unpacked
(* False *)

Testing VectorQ[list, IntegerQ]:

VectorQ[packed, IntegerQ]
VectorQ[unpacked, IntegerQ]
VectorQ[nonint, IntegerQ]
(*
  True
  True
  False
*)

Comparing the timings with two other methods (the timing function timeAvg is given below):

VectorQ[packed, IntegerQ] // timeAvg
VectorQ[unpacked, IntegerQ] // timeAvg
VectorQ[nonint, IntegerQ] // timeAvg
(*
  2.64695*10^-7
  0.0319406
  0.00318189
*)

MatchQ[packed, {___Integer}] // timeAvg
MatchQ[unpacked, {___Integer}] // timeAvg
MatchQ[nonint, {___Integer}] // timeAvg
(*
  9.5835*10^-7
  0.157701
  0.065558
*)

And @@ (IntegerQ /@ packed) // timeAvg
And @@ (IntegerQ /@ unpacked) // timeAvg
And @@ (IntegerQ /@ nonint) // timeAvg
(*
  2.782322
  2.858287
  2.540433
*)

You may note that on packed arrays VectorQ and MatchQ take virtually no time. In fact, it's the same amount of time no matter what the size. This is because a packed array is a special efficient internal representation of an array. In particular it has to be an array of all the same type of number (only Integer, Real, and Complex are allowed). So checking the type is easy. See What is a Mathematica packed array?

---

The site-standard timeAvg function:

SetAttributes[timeAvg, HoldFirst]
timeAvg[func_] := 
  Do[If[# > 0.3, Return[#/5^i]] & @@ 
    AbsoluteTiming@Do[func, {5^i}], {i, 0, 15}];

Use Map, abbreviated /@. For example:

  lis = {4, 5/2, 3., 6/3, Pi};
  IntegerQ /@ lis               (* or Map[IntegerQ, lis] *)

(* {True, False, False, True, False} *)