How to access ephemeral storage of EC2 instance?
You are correct that the describe-instances API doesn't include information about ephemeral volumes. Once an instance is created, the only way to know that it has such volumes is available from the metadata service that is accessible from the instance itself.

According to User Guide for Linux Instances, the mapped ephemeral volumes are shown in instance metadata.

Since you created the instances using Chef, another option would be to check the Chef state database - most probably the information about a created instance includes the ephemeral device information. But this method highly depends on your Chef version and the cookbook you use to create the instances, so YMMV.