I have had a couple of upside down catfish (Synodontis nigriventris) for several years now and i have always assumed one of them was female because it was much plumper then the rest. Up until a few days ago they shared a tank with 8 tiger barbs so i rarely saw them. Now that the barbs have been transferred the one that has always appeared plumper has at least doubled in size. she looks like she is ready to pop. Here is a pic:
