Well I looked in a Delta flight schedule I picked up at the airport last month, and it doesn't give any way to tell the difference between flights on the 767-300 and 767-300ER. In the front it lists the 767-300 as having 5 possible seating configurations of 10+28+174 (212); 10+28+165 (203); 24+228 (252); 48+147 (195); and 48+142 (190). Their website doesn't specify, either.
If it's making you nuts to find out, I guess you could try calling their reservations and ask them if they know whether it's a 300 or 300ER. And if they don't know, you might be able to figure it out by elimination by asking them what the capacity for that flight is and match it up with the numbers above. I feel certain the first 2 or 3 would be the ER since the three class configuration is more typical of international service aircraft, and the 2 class configuration is usually for domestic routes.