I noticed someone mentioned jQuery, but I didn't know there was an isArray() function. It turns out it was added in version 1.3.
jQuery implements it as Peter suggests:
isArray: function( obj ) {
return toString.call(obj) === "[object Array]";
},
Having put a lot of faith in jQuery already (especially their techniques for cross-browser compatibility) I will either upgrade to version 1.3 and use their function (providing that upgrading doesn’t cause too many problems) or use this suggested method directly in my code.
Many thanks for the suggestions.