To list the things associated with the specified certificate, along with the attachment type, run the list-principal-things-v2 command. The attachment type refers to how the certificate is attached to the thing.

$ aws iot list-principal-things-v2 \
    --principal "arn:aws:iot:us-east-1:123456789012:cert/2e1eb273792174ec2b9bf4e9b37e6c6c692345499506002a35159767055278e8"

The output can look like the following.

{
    "PrincipalThingObjects": [
        {
            "thingPrincipalType": "NON_EXCLUSIVE_THING",
            "thing": "arn:aws:iot:us-east-1:123456789012:thing/thing_1"
        }, 
        {
            "thingPrincipalType": "NON_EXCLUSIVE_THING",
            "thing": "arn:aws:iot:us-east-1:123456789012:thing/thing_2"
        }

    ]
}

For more information, see ListPrincipalThingsV2 from the AWS IoT Core API Reference.