BZ, I can tell because of their habits – where they perch, how they come to eat when I whistle for them, and because I have never been WITHOUT a male, ever. My current male I truly believe to be a son of my first male, simply because his habits were VERY similar and he has always been here, but he does have a few different habits from the previous. Coloring & size also can be telltale, but they do change colors somewhat, especially when they molt in the fall. Of course, I cannot say with 100% accuracy that they are the same, but I would say 99.9%. I have had about 4 or 5 different females – one was killed by a cat, 2 just disappeared while incubating probably died or got killed, etc. I knew the females were gone when they did not show up with the male for several days. My male had another female within a few days after she disappeared this April. Just thought of another thing for me – I have a cage feeder and a new bluebird would not come as readily to one of those – they need time to get accustomed to those, and my males have never refused to go inside the cage feeder – another good reason I believe my current male to be a descendant from my first male of 2008, 9 seasons ago. Make any sense?