First, you need to know what the original objectives were for having the ontology. The more specific the objectives, the easier it is to test whether they have been met. If you never had any objectives (or requirements), it might be easier to declare success, but harder to defend what that means.
Whatever the specific objectives might be, some general things to look at include correctness, completeness, elegance and understandability. Correctness means that the classes, properties and axioms accurately characterize the subject matter being modeled. A good place to start is to run an inference engine to see if there are any logical inconsistencies. Completeness means that the ontology has the right coverage of subject matter – no important concepts are left out. Determining what should be in or out is a scoping issue that strongly depends on the ontology objectives. Elegance is the other side of the completeness coin: are there things in the ontology that are not needed? Importantly, this includes avoiding redundancy and complexity. Understandability is about the ontology being easy to learn and use. There must be high level overviews for those who don’t need the details, and an easy way to drill down to the specifics for those who do.