On the order details page
Customers (and guests via the order-received link) can see their own signature on the order details page, in the position you chose under General Settings.

In the WordPress admin
Open WooCommerce → Orders, click into any signed order, and scroll to the billing address section the signature image appears there.

In the Media Library
Every signature is saved as a normal image attachment, named signature_{order_id}.png (or .jpg), so you can also find it under Media → Library.
