slot-boundp

(slot-boundp instance 'slot-name)

Whether the named slot of the instance holds a value: nil when the instance's class has no such slot, when the slot was written with no :initform and no initarg supplied it, or after slot-makunbound; t otherwise. Reading an unbound slot with slot-value or an accessor signals unbound-slot, whose cell-error-name is the slot and whose unbound-slot-instance is the object.

On the JVM and WASM compilers the slot name must be a literal quoted symbol, like slot-value; a runtime-computed slot name works on the interpreter only.