Well, you could always perform some kind of numeric operation on your array element and see if performs as you'd expect. If it doesn't, it's probably a string. Try this for example:

if( abs( $array[ 0 ] ) != 0 ) {
   # the absolut value of this scalar is not 0, so it's probably not a
+ string
}
[download]

This works with floats as well as really large numbers (like "5.4e+10"), but beware: the absolute value of 0 is 0, so that's a false negative. I'll leave the handling of 0 as an excercise for the reader :)

Cheers,
--Moodster