This assumes you are already created an app in https://developers.facebook.com. If not, check out my article Getting Started with Facebook Graph API.

You will first need to get the User Access Token. Check out my article Facebook Graph API - Get Access Token. For example, 

1. Go to https://developers.facebook.com and select My Apps.
2. Select your App.
3. Select Tools > Graph API Explorer.

By default, the query should be setup to GET v18.0/me?fields=id,name. Select Submit and this should return your User ID and User Access Token, something like this.

EAANROE66DrsBOy1TiJVQPEPEZAgo2Iap7QzdKEGRfJBW30drpIpkprZB4N5MRxDfZCoLh4OQZCCDgPqVxFEO39YDTIZCmChr9b3QcDr5MxPSjxvnY4Q1uPpMZCgG6Tr0FsZAW2vJD3qo5LpYZCu2JE2VhBUZCAPnZCCnysHlvca9QQ9BS2cvXxccHmUGPEKml4ncE0BYrfh7bI9ZAL9M7sLBgZDZD

Then submit a GET request to the /<group id> endpoint and something like this should be returned.

{
  "name": "my Group",
  "privacy": "OPEN",
  "id": "123456789012345"
}

Or like this, using curl.

curl \
--request GET \
--url "https://graph.facebook.com/v18.0/<group id>?access_token=<user access token>"