The critics declared the play a 'winner'.

What kind of complement is indicated by the bolded word? object complement

predicate nominative

direct object

predicate adjective

say the subject; say the verb; ask whom? or what?

the critics declared what? the play

can you replace the verb with a form of is or an = sign leaving the sentence meaning the same?

The critics are the play. The critics = the play.

[if the meaning stays the same, then you have a predicate adjective or predicate nominative (noun) ]

This sentence changes meaning with this test, so play is the direct object.

Can you put "to be" in front of the following noun?

The critics declared the play to be a winner.

yes--so "a winner" is the objective complement---it identifies (or as an adjective would describe) the direct object.