Behavior-testing the form of the instances - GNU Smalltalk Library Reference

instSize: Answer how many fixed instance variables are reserved to each of the receiver's instances
isBits: Answer whether my instances' variables are immediate, non-OOP values.
isFixed: Answer whether the receiver's instances have no indexed instance variables
isIdentity: Answer whether x = y implies x == y for instances of the receiver
isImmediate: Answer whether, if x is an instance of the receiver, x copy == x
isPointers: Answer whether the instance variables of the receiver's instances are objects
isVariable: Answer whether the receiver's instances have indexed instance variables