Hmm well, I always thought that Bara is the art made to gays and usually is made by gay people (it isn't always the case) most differences with Yaoi or muscular male, is basically that yaoi use more "beautiful, fine" traits with their males while bara use more "tough, raw, manly" traits with their males, for saying that yaoi usually the muscles looks more "soft" than in bara or the face is more "cute".
For other hand, while bara must have muscular males, it doesn't mean all muscular males belong to bara, you can have a muscular male in your work but that doesn't mean you made them for the gays and you wanted to show a badass big dude or just a strong dude, what I use to identify when a muscular male is just that, is when the artist don't make the male body "shine", like yeah he is muscular but the body isn't the main "protagonist" or the body just show he is big but the muscles aren't that defined, I think a better world would be eroticized.
Other thing would be bara artist... Bara usually is by and for gays, but there are works where the artist is a woman and still their works show a strong dude that looks like they were taken out from a bara, big strong body that is desplayed, tough traits, veiny body and in cases even hairy. In those cases I also apply what I say and if the artist make them to be seeing, then I think it also apply as bara.
But I can't deny that seeing some works with the "bara" tag, there are some that don't qualify as bara, sometimes is used more as the "gay" tag because it show a couple but how they doesn't look like your usual yaoi (beautiful) they use bara that is more for "manly" dudes.
So in a nutshel: Bara, for me, are works that show muscular male be alone or in group with tough/manly traits (being be muscular, tall, hairy, veiny, old, raw lines, etc.) that is target is usually the gays. It differ from Yaoi by keeping away from the soft/beauty trait that Yaoi usually use on their works.