Although you cannot request the class of a particular object, you can probably establish what class it belongs to with the typeof operator.
ECMA 262 edition 2 - section - 7.4.3
ECMA 262 edition 2 - section - 11.4.3
ECMA 262 edition 3 - section - 7.5.3
ECMA 262 edition 3 - section - 11.4.3