It's very easy to tell and believe me I know. I spent literally hours looking at some decent size fish tailing over grass beds a couple of months ago and I was originally certain they were drum. As I studied these fish I realized they were not drum and I got to looking at the details and I found the easily spotted difference. The mullet have tails that are sharply pointed and drum have rounded tails. These mullet were large too. They could have passed as year old drum of 12-16". Next time you see them look for the sharply pointed tails and you will know they are mullet for sure.