This assumes your last subscription was in the last 7 days and you know when the money gets withdrawn. Some subscriptions are not monthly, so you end up having to search for a specific subscription you know of to click on to get to the secret My preapproved payments page.
Of course, the polite way of handling subscriptions would be having a link to this page in the menu. Sort of like how there are links to all of the other things you can do.
Does this feature honestly not seem hidden to you compared to the other things you can do on PayPal?
You may just end up pulling all you hair out before you find a way to do so.