I cannot speak for Bernd, – but I find that the spike in the tail becomes distincty longer, and the two pectorial fins as well, much longer. Apart from that, theres also a question of size and bodyshape, but this is only when the fish has a certain age. And I think it is rather difficult.
This thread and this question made me think of a video film which I made a couple of years ago, – because theres a female linkei in it, which stands still for almost minutes 🙂 .. so maybe she is a good example. I know it was a female. You can find her at 5.35.
I apologize for uploading this quite long video here in a thread which is not really about this issue, – but I hope you will forgive me. The film was made for fun one day when I thought I would teach the world about the beautiful paros. It lies on youtube, but I dont know how many people really see it. There may be some ‘mistakes’ in it, I have sinced learned a few things more I guess.