You probably have to assume they are not. A ceiling fan box has to have sufficient structural connection to hold the weight. Unless someone was preplanning a fan installation, chances are that it is not.

The box should be a metal one and securely mounted to the ceiling structure through its back, not a side bracket.
There are some plastic boxes that are made for lighter weight fans, but I would look for a metal box if I weren't installing it myself.

Boxes "suitable for fan support" will have that statement stamped inside the box.

Thanks so much for the responses.

There are fan brace boxes available that can be installed through the existing hole in the ceiling.

They unscrew to anchor themselves into the joists.