How are indexes represented in an ERD?
They don't tend to be shown in ERDs. An ERD focuses on the Entities and their Relationships, but an index is a copy of the data from one (or potentially more) of the entities, created to assist in the execution of queries. While it's possible that an index could be unique and therefore contribute to the database design, they are not typically shown on ERDs.

...which is a shame, because in my experience, a database design should involve careful acknowledgement of the indexes, as they can make-or-break the effectiveness of a database system.