Go to the Amazon site, sign in, and look (top right) at where it says "Hello XXXX Account and lists" Under that you will find "Your Prime Membership"
If under that it invites you to "Start Your Amazon Prime Free Trial Membership" then you DON'T have it already.
If you DO have the free trial, it will tell you how long your free trial has to go - and you can cancel it before it is no longer free.
If you are now paying, it should tell you when your next subscription is due (I have managed so far to avoid letting it get this far, so I am not sure whether it does tell you this) but your previous payment will be visible on your bank statement.