cURL is a command line tool for making HTTP requests. Since Cloud Vision API is accessed over REST, we can use curl as a client for it. You can download cURL for your OS from here: https://curl.haxx.se/download.html. You can test the installation by running the following command: 

$ curl --version
curl 7.50.0 (x86_64-apple-darwin15.4.0) libcurl/7.50.0 OpenSSL/1.0.2h zlib/1.2.8
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP UnixSockets

To test whether cURL is working as expected, run the following command:

$ curl https://reqres.in/api/users

This hits a public-hosted free mock REST API service named Req | Res (https://reqres.in/) and will return a bunch of users. Your output should look something like this:

//SNIPP SNIPP
{
    "page" : 1,
    "data" : [
       {
          "first_name" : "George",
          "avatar" :  "https://s3.amazonaws.com/uifaces/faces/twitter/calebogden/128.jpg",
          "id" : 1,
          "last_name" : "Bluth"
       },
       {
          "id" : 2,
          "avatar" :     "https://s3.amazonaws.com/uifaces/faces/twitter/josephstein/128.jpg",
         "last_name" : "Weaver",
         "first_name" : "Janet"
       },
       {
         "last_name" : "Wong",
         "id" : 3,
         "avatar" : "https://s3.amazonaws.com/uifaces/faces/twitter/olegpogodaev/128.jpg",
         "first_name" : "Emma"
       }
    ],
    "total_pages" : 4,
    "per_page" : 3,
    "total" : 12
}
//SNIPP SNIPP

If you are not comfortable with command line-based interaction, you can use a tool such as Postman.