I lost the signing secret for my webhook. Can I see it again?

No. The signing secret is shown only once, right after you click Create webhook. It is never shown again in the list.

To get a new one:

1. Go to Plugins → Webhooks.
2. Under Add a webhook, enter the same address, pick the same events, and click Create webhook.
3. Copy the new Signing secret and update it in your app.
4. Under Subscriptions, click Delete on the old webhook. There is no confirmation step, so make sure you pick the right one.

The same steps apply if you need to change the address or the events of a webhook, because an existing webhook cannot be edited.