Is there a command/subcommand that can be passed to the aws utility that can 1) verify that the credentials in the ~/.aws/credentials file are valid, and 2) give some indication which user the credentials belong to? I'm looking for something generic that doesn't make any assumptions about the user having permissions to IAM or any specific service.

The use case for this is a deploy-time sanity check to make sure that the credentials are good. Ideally there would be some way to check the return value and abort the deploy if there are invalid credentials.

Use GetCallerIdentity:
aws sts get-caller-identity

Unlike other API/CLI calls it will always work, regardless of your IAM permissions.

You will get output in the following format:

{
    "Account": "123456789012", 
    "UserId": "AR#####:#####", 
    "Arn": "arn:aws:sts::123456789012:assumed-role/role-name/role-session-name"
}

Exact ARN format will depend on the type of credentials, but often includes the name of the (human) user.

It uses the standard AWS CLI error codes giving 0 on success and 255 if you have no credentials.