You are going to kick yourself on this one, but dont' feel too bad. It's so obvious it's easy to overlook.
Two places actually. First, in the screen when you are replying on the left side. (Note you don't actually have to send a rply, you can just hit rply, or accept, or refuse to get to that screen and then cancel back without sending a message.)
Second, in the diplomacy screen (Empires) below the empire offering you the treaty it will say what level of treaty they offered. (This one however disapeears after you reply to the message.)
Geoschmo
[ 16 April 2002: Message edited by: geoschmo ]