This assumes you have already configured the aws command line tool. If not, check out my article on Getting Started with the AWS CLI.

An EC2 instance can only be associated with one instance profile. The aws ec2 describe-iam-instance-profile-associations command to determine if one of your EC2 instances is associated with an instance profile.

~]$ aws ec2 describe-iam-instance-profile-associations
{
    "IamInstanceProfileAssociations": [
        {
            "AssociationId": "iip-assoc-04a03786337a32660",
            "InstanceId": "i-0a3ea97aa2383de58",
            "IamInstanceProfile": {
                "Arn": "arn:aws:iam::123456789012:instance-profile/my-instance-profile",
                "Id": "AIPA2MITL76GNDM4CZFIM"
            },
            "State": "associated"
        }
    ]
}