// ранее: var something = ... ;
if( Array.isArray( something) console.log( 'isArray: true' );
if( something instanceof Array) console.log( 'instanceof: true' );
if( Object.prototype.toString.call( something ) === '[object Array]' ) console.log( 'toString: true');
if( something.constructor === Array) console.log( 'constructor: true');