$ aws cognito-idp admin-get-user --user-pool-id COGNITO_USER_POOLS_ID --username USERNAMEでいけます。

User does not existが返ってきたらユーザーは存在しない（or退会済）です。

$ aws cognito-idp admin-get-user --user-pool-id COGNITO_USER_POOLS_ID --username USERNAME

An error occurred (UserNotFoundException) when calling the AdminGetUser operation: User does not exist.

成功したらこんな感じ。

$ aws cognito-idp admin-get-user --user-pool-id COGNITO_USER_POOLS_ID --username hideokamoto
{
    "Username": "hideokamoto", 
    "MFAOptions": [
        {
            "DeliveryMedium": "SMS", 
            "AttributeName": "phone_number"
        }
    ], 
    "Enabled": true, 
    "UserStatus": "CONFIRMED", 
    "PreferredMfaSetting": "SMS_MFA", 
    "UserCreateDate": 1484806199.008, 
    "UserAttributes": [
        {
            "Name": "sub", 
            "Value": "b3e56722-c100-4fec-9d0d-a1311481a0f0"
        },