Hey Dhara,
You have a couple of options here.
First, you can trigger the webhook for the topic you subscribed to by completing the associated action. ie: you've subscribed to 'products/update' so you change the title of a product to send a webhook to the address you've tied to the webhook subscription when you created it using the Admin API.
Alternatively, if you are using a development shop for testing, you can use the notification panel in the shop admin. By going to Settings -> Notifications -> Webhooks, you can create a webhook for any of the provided topics pointing to whatever address you like. You can then click the "Send test notification" button to send a webhook with the corresponding payload to the address you provided.
Some partners like to make use of online tools such as RequestBin where they can point their webhooks to for easy capture and analysis.