When it comes to German Blue Ram's, the only surefire way to tell is the pink belly, especially around breeding time. I've been told to look at the fins just like the previous post but you can still be fooled by that. I have a couple female rams, an EBR and a GBR that the females look like males if you just look at the fins. One of them was sold to me by my LFS as a male before I got home and realized it so don't trust just looking at the fins unless you really know what you're looking for. The Electric Blue's are harder to determine the sex as the pink is all but covered up at which point you have to look at the fins and make the determination. After having Ram's long enough it gets really easy to tell.