Go to the trade screen like you were trying to start a new trade. At the bottom, you'll see New and Active. It pulls up New by default. If you click on the Active icon, it will pull up all active trades and agreements. If they are still within the 20 turn term of the agreement, a number shows up next to the deal. If past the 20 turn term, they won't show a number, and you can click on the agreement to renegotiate or cancel it.
An easier way (IMO) is to go to preferences and set preferences to 'Always renegotiate'. That way you don't have to remember when things are expiring. Peace Treaties are the only ones that auto-renew if you check that preference.