Yeah with lugs a way to have some idea of the structural integrity is to braze from one size and pull the filler to the other. This way you know the joint is pretty good. But really the lugs hides the joint so the only way to know is to slice it up.

You can also make some practice joints and slice them up, these days it's what I am doing. Good thing I did it 'cause I could not pull the brass in the BB shell, and upon slicing it up the penetration was quite miserable.

With fillets you have a better idea by looking at the finished joint, but again perhaps your mitres moved a bit, or you didn't heat sufficiently and the internal fillet is bad...

Or I don't know some fatigue tests like they do in large productions? That would require some machinery...