Requestb.in provides you with a URL on which you can check if webhooks for DLR are working properly.
You can follow these steps to see how you will receive the data on your URL:
1. Go to https://requestbin.fullcontact.com
2. Click on Create a RequestBin
3. Copy the URL
4. Paste this URL in the webhooks, and send an SMS
5. Click on the bubble to view the data received on this URL